Document:

<PAGE>

                                                                 Exhibit 10.8.1

[*] IMPORTANT NOTE: Certain material, indicated by an asterisk ("*"), has been
omitted from this document pursuant to a request for confidential treatment. The
omitted material has been filed separately with the Securities and Exchange
Commission.

                      FIRST AMENDMENT TO CREDIT AGREEMENT

      This FIRST AMENDMENT TO CREDIT AGREEMENT (this "Amendment") is dated
effective as of July 28, 1999 (the "Amendment Date") unless otherwise expressly
stated herein, is by and among NET-TEL CORPORATION (the "Borrower"), a Florida
corporation, NET-TEL COMMUNICATIONS, INC. ("Holdings"), a Delaware corporation,
each of the undersigned Lenders (as hereinafter defined) and NORTEL NETWORKS
INC., a Delaware corporation, as administrative agent for itself and the other
Lenders (in such capacity, together with its successors in such capacity, the
"Administrative Agent").

                                    RECITALS:

      A. Pursuant to that certain Credit Agreement dated as of July 28, 1999 (as
the same may be amended, modified, renewed, extended, restated or supplemented
from time to time, the "Credit Agreement") by and among the Borrower, each of
the lending entities which is a party thereto or which may from time to time
become a party thereto as a lender or any successor or assignee thereof
(individually, a "Lender" and, collectively, the "Lenders"), and the
Administrative Agent, the Lenders agreed to provide, with the assistance of the
Administrative Agent, (i) a $120,000,000 advancing term loan facility to finance
a portion of the Borrower's costs to purchase Nortel Networks Goods and Services
(as defined in the Credit Agreement) and to finance certain Eligible Third-Party
Expenses (as defined in the Credit Agreement), (ii) a $10,000,000 term loan
facility to finance certain of the Borrower's working capital needs and for
general corporate purposes, and (iii) a $10,000,000 revolving loan facility to
finance certain of the Borrower's working capital needs and for general
corporate purposes and in the ordinary course of business.

      B. The Borrower, the Administrative Agent and the Lenders desire to amend
the Credit Agreement upon the terms and conditions contained herein.

                                   AGREEMENTS:

      NOW, THEREFORE, in consideration of the premises and mutual covenants and
agreements contained herein and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
hereby agree as follows:

      1. Terms Defined. Unless otherwise defined or stated in this Amendment,
each capitalized term used in this Amendment has the meaning given to such term
in the Credit Agreement (as amended by this Amendment).

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 1
<PAGE>

      2. Amendments.

      (a) Effective as of February 21, 2000, the term "Maximum Financed Amount
of Eligible Third-Party Expenses" and the definition thereof set forth in
Section 1 of the Credit Agreement are hereby amended and restated to read in
their entirety as follows:

            "Maximum Financed Amount of Eligible Third-Party Expenses" means, as
      of any date of determination, the lesser of (a) an amount equal to the
      aggregate amount of Eligible Third-Party Expenses or (b) *% of the sum of
      (without duplication) (i) the aggregate amount paid by the Borrower to
      Nortel Networks and/or Nortel Networks Corporation to purchase Nortel
      Networks Goods and Services for the Network plus (ii)*.

      (b) Subclause (g)(iv) of the definition of the term "Permitted Liens"
contained in Section 1.1 of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (iv) the Debt secured by all such Liens, when aggregated with the
      Debt secured by all purchase-money Liens at any time outstanding (whenever
      incurred or created) and all Liens in connection with any conditional sale
      or other title retention agreement or Capital Lease Obligation existing as
      of the Closing Date or at any other time, shall not exceed (A) $* at any
      time outstanding in the aggregate prior to March 27, 2000, and (B) $* at
      any time outstanding in the aggregate on and after March 27, 2000.

      (c) Section 2.7 of the Credit Agreement is hereby amended by adding the
following Section 2.7(g), which Section shall read in its entirety as follows:

          (g) Maximum Financed Amount of Eligible Third-Party Expenses. *

      (d) Section 2.11(a) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

          (a) Subject to Section 13.12, the Borrower shall, commencing on *,
      and for all periods thereafter, pay to the Administrative Agent for the
      account of each applicable Lender a commitment fee on the daily average
      unused or unfunded amount of each of such Lender's Term Loans A
      Commitments and Revolving Loans Commitments (as the same may be terminated
      or reduced pursuant to Section 2.13), for the period from and including
      the date upon which such Lender (or the date its predecessor in interest
      with respect to such Commitments assigned to such Lender as to which a
      commitment fee has not previously been paid during the applicable period)
      became a party hereto to but excluding the Term Loans A Commitment
      Termination Date or the Revolving Loans Commitment Termination Date, as
      the case may be, at the rate of * percent (*%) per annum based on a 360
      day year and the actual number of days elapsed, which accrued commitment
      fees shall be payable in arrears on each Quarterly Date and on the Term
      Loans A Commitment Termination Date or the Revolving Loans Commitment
      Termination Date, as the case may be.

* Confidential treatment requested.  The redacted material has been separately
  filed with the Securities and Exchange Commission.

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 2
<PAGE>

      (e) Section 8.1(b) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (b) Quarterly Financial Statements. As soon as available, and in any
      event within 45 days after the end of each of the first three quarters of
      each fiscal year of the Borrower, beginning with the fiscal quarter ending
      June 30, 1999 unless otherwise expressly stated herein, either (i) a copy
      of the Form 10-Q (including all financial statements contained therein)
      filed by Holdings as of the end of and for such fiscal quarter then ended,
      together with consolidating schedules for each of Holdings and its
      Subsidiaries (including, without limitation, the Borrower) with respect to
      each of the financial statements contained therein, or (ii) a copy of the
      unaudited consolidated balance sheet of the Borrower and its Subsidiaries
      as of the end of such quarter and the related consolidated statements of
      income or operations, shareholders' equity and cash flows and, commencing
      with the fiscal quarter of the Borrower ending December 31, 1999,
      quarterly operating budgets for the period commencing on the first day and
      ending on the last day of such quarter, together with unaudited
      consolidating schedules for the Borrower and its Subsidiaries with respect
      to each of such financial statements and quarterly operating budgets, in
      each case, commencing with the fiscal quarter of the Borrower ending
      December 31, 1999, setting forth in comparative form the information or
      figures and quarterly operating budget figures, respectively, for the
      corresponding period of the preceding fiscal year, and certified by an
      appropriate Responsible Officer of the Borrower as fairly presenting, in
      accordance with GAAP, the financial position and the results of operations
      of the Borrower and its Subsidiaries (except for year-end adjustments and
      the absence of financial statement footnotes required by GAAP);

      (f) Section 8.1(c) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (c) Compliance Certificate. Commencing with the fiscal quarter of
      the Borrower ending *, concurrently with the delivery of each of the
      financial statements referred to in Sections 8.1(a) and 8.1(b), a
      Compliance Certificate of a Responsible Officer of the Borrower
      substantially in the form of Exhibit D hereto, appropriately completed,
      stating that, to the best of such officer's knowledge, no Default has
      occurred and is continuing or, if a Default has occurred and is
      continuing, stating the nature thereof and the action that has been taken
      and is proposed to be taken with respect thereto;

      (g) Section 8.1(h) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (h) Insurance. Commencing with the fiscal year of the Borrower
      ending December 31, 2000, within 60 days prior to the end of each fiscal
      year of the Borrower, a report in form and substance reasonably
      satisfactory to the Administrative Agent summarizing all material
      insurance coverage maintained by the Borrower and its Subsidiaries as of
      the date of such report and all material

* Confidential treatment requested.  The redacted material has been separately
  filed with the Securities and Exchange Commission.

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 3
<PAGE>

      insurance coverage planned to be maintained by such Persons in the
      subsequent fiscal year;

      (h) Section 8.1(j) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (j) Business Plan, etc. Commencing with the fiscal year of the
      Borrower ending December 31, 2000, not later than 15 days prior to the end
      of each fiscal year, an update of the Business Plan for the immediately
      succeeding fiscal year in reasonable detail generally consistent with the
      form and substance of the Business Plan provided to the Administrative
      Agent on or before the Initial Funding Date, which update shall reflect
      the corresponding information for the prior year; and, promptly upon the
      Borrower's preparation thereof, any proposed amendment, modification or
      supplement to the Business Plan;

      (i) Section 8.1(p) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (p) Accounts Receivable and Payable. Commencing with the fiscal
      quarter of the Borrower ending December 31, 1999, as soon as available and
      in any event within (i) 60 days after the end of the fiscal quarter of the
      Borrower ending December 31, 1999 and (ii) 45 days after the end of each
      fiscal quarter thereafter, an aged trial balance of all then-existing
      Receivables and all then existing accounts payable of the Borrower and its
      Subsidiaries;

      (j) Section 8.1(r) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (r) Quarterly Report as to Key Business Statistics. Commencing with
      the fiscal quarter of the Borrower ending December 31, 1999, as soon as
      available and in any event within (i) 60 days after the end of the fiscal
      quarter of the Borrower ending December 31, 1999 and (ii) 45 days after
      the end of each fiscal quarter thereafter, reports as to key business and
      operational statistics of the Borrower and its Subsidiaries, including,
      without limitation, reports as to the number of customers, number of Sales
      Representatives and such related information as the Administrative Agent
      may reasonably request from time to time;

      (k) Section 8.1(s) of the Credit Agreement is hereby amended and restated
to read in its entirety as follows:

            (s) Borrowing Base Reports and Agings. Commencing with the fiscal
      quarter of the Borrower ending December 31, 1999, as soon as available and
      in any event within (i) 60 days after the end of the fiscal quarter of the
      Borrower ending December 31, 1999 and (ii) 15 days after the end of each
      fiscal quarter thereafter, and, in any event concurrently with the making
      of each Revolving Loan hereunder and from time to time upon the request of
      the Administrative Agent after the occurrence of a Default, (i) a
      Borrowing Base Report duly completed and (ii) an aged

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 4
<PAGE>

      trial balance of all then-existing Receivables and all then existing
      accounts payable of the Borrower and its Subsidiaries; and

      (l) Section 8.17 of the Credit Agreement is hereby amended to change the
date "September 30, 1999" contained therein to the date "March 27, 2000".

      (m) A new Section 13.24 is hereby added to the Credit Agreement, which
Section 13.24 shall read in its entirety as follows:

            Section 13.24 Miscellaneous Matters. Notwithstanding anything to the
      contrary contained in this Agreement:

                  (a) the Loan Documents required to be executed by the
            Borrower's Subsidiary corporation organized under the laws of the
            State of Virginia shall not be required to be executed or delivered
            by such corporation until on or before March 27, 2000, and the
            Borrower shall not be required to pledge to the Administrative Agent
            the shares of Capital Stock of such corporation until on or before
            March 27, 2000;

                  (b) the Borrower shall not be required to deliver to the
            Administrative Agent a landlord subordination or waiver (as required
            in accordance with Section 5.4) with respect to the Borrower's
            Washington, D.C. switch location until on or before March 27, 2000;
            and

                  (c) the Borrower shall not be obligated to deliver to the
            Administrative Agent an endorsement to the issuance policies
            (required to be maintained under the Credit Agreement) naming the
            Administrative Agent as additional insured and loss payee until on
            or before March 27, 2000.

      (n) Exhibit C to the Credit Agreement is hereby amended and restated to
read in its entirety as set forth in First Amendment Exhibit A to this
Amendment.

      (o) Schedule 10.8 of the Credit Agreement is hereby amended to delete the
minimum Gross Revenues requirements for the calendar quarters (and only for the
calendar quarters) ended September 30, 1999 and December 31, 1999.

      (p) Schedule 10.9 of the Credit Agreement is hereby amended to delete the
minimum EBITDA requirements for the calendar quarters (and only for the calendar
quarters) ended September 30, 1999 and December 31, 1999.

      (q) Schedule 10.12 of the Credit Agreement is hereby amended to delete the
minimum requirement for Average Monthly Gross Revenues Per Sales Representatives
for the calendar quarters (and only for the calendar quarters) ended September
30, 1999 and December 31, 1999.

            3. Conditions Precedent. The effectiveness of this Amendment is
subject to the satisfaction of each of the following conditions precedent, all
of which conditions precedent must

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 5
<PAGE>

be satisfied on or before February 29, 2000 (unless any one or more of such
conditions precedent are waived in writing by the Administrative Agent):

            (a) The Administrative Agent shall have received all of the
following, each dated (where applicable and unless otherwise indicated) the date
of this Amendment, in form and substance satisfactory to the Administrative
Agent:

                  (i) Amendment Documents. This Amendment as executed by the
      parties hereto and an Amended and Restated Administrative Agent's Letter,
      in form and substance satisfactory to the Administrative Agent, executed
      by the Borrower and Nortel Networks (collectively, the "Amendment
      Documents");

                  (ii) Resolutions. Resolutions of the Board of Directors of the
      Borrower certified by its Secretary or an Assistant Secretary which
      authorize the execution, delivery and performance by the Borrower of this
      Amendment and the other Amendment Documents to which it is or is to be a
      party;

                  (iii) Insurance. A report in form and substance reasonably
      satisfactory to the Administrative Agent summarizing all material
      insurance coverage maintained by the Borrower and its Subsidiaries as of
      the date of such report and all material insurance coverage planned to be
      maintained by such Persons in the fiscal year of the Borrower ending
      December 31, 2000;

                  (iv) Business Plan, etc. An update of the Business Plan for
      the fiscal year of the Borrower ending December 31, 2000 in reasonable
      detail generally consistent with the form and substance of the Business
      Plan provided to the Administrative Agent on or before the Initial Funding
      Date, which update shall reflect the corresponding information for the
      prior year; and, promptly upon the Borrower's preparation thereof, any
      proposed amendment, modification or supplement to the Business Plan;

                  (v) Fees, Costs and Expenses. If so requested by the
      Administrative Agent, all fees, costs and expenses (including, without
      limitation, attorneys' fees and expenses) incurred by the Administrative
      Agent and each of the Lenders incident to this Amendment or required to be
      paid in accordance with Section 13.1 of the Credit Agreement, to the
      extent incurred and submitted to the Borrower, shall have been paid in
      full by the Borrower;

            (b) The representations and warranties contained herein and in all
other Loan Documents, as amended hereby, shall be true and correct as of the
date hereof as if made again on and as of the date hereof (except if and to the
extent that such representations and warranties are or were expressly made only
as of another specific date); and

            (c) No Event of Default shall have occurred and be continuing
(immediately after giving effect to this Amendment).

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 6
<PAGE>

Upon the satisfaction of all conditions precedent set forth in this Paragraph 3
and upon request of confirmation of such satisfaction made by the Borrower to
the Administrative Agent, the Administrative Agent shall confirm such
satisfaction in writing to the Borrower.

      4. Governing Law. THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS SHALL BE
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK
(WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES) AND APPLICABLE LAWS OF THE U.S.

      5. Counterparts. This Amendment may be executed in any number of
counterparts, all of which when taken together shall constitute one agreement,
and any of the parties hereto may execute this Amendment by signing any such
counterpart.

      6. No Oral Agreements. THIS AMENDMENT, TOGETHER WITH THE CREDIT AGREEMENT
AND THE OTHER LOAN DOCUMENTS AS WRITTEN, REPRESENT THE FINAL AGREEMENTS BETWEEN
AND AMONG THE PARTIES HERETO RELATING TO THE SUBJECT MATTER HEREOF AND MAY NOT
BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL
AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN (A)
THE BORROWER OR ANY OTHER LOAN PARTY AND (B) THE ADMINISTRATIVE AGENT AND/OR ANY
LENDER.

      7. Agreement Remains in Effect; No Waiver. Except as expressly provided
herein, all terms and provisions of the Credit Agreement and the other the Loan
Documents shall remain unchanged and in full force and effect and are hereby
ratified and confirmed. No waiver by the Administrative Agent or any Lender of
any Default or Event of Default shall be deemed to be a waiver of any other
Default or Event of Default. No delay or omission by the Administrative Agent or
any Lender in exercising any power, right or remedy shall impair such power,
right or remedy or be construed as a waiver thereof or an acquiescence therein,
and no single or partial exercise of any such power, right or remedy shall
preclude other or further exercise thereof or the exercise of any other power,
right or remedy under the Credit Agreement, the Loan Documents or otherwise.

      8. Representations and Warranties. All representations and warranties made
in this Amendment or any other Loan Document shall survive the execution and
delivery of this Amendment and the other Loan Documents, and no investigation by
the Administrative Agent or any closing shall affect the representations and
warranties or the right of the Administrative Agent and the Lenders to rely upon
such representations and warranties.

      9. Ratification of Obligations under the Loan Documents. Each of the
undersigned Loan Parties acknowledges and consents to the terms of this
Amendment and agrees that all of its indebtedness, liabilities and obligations
under the Loan Documents to which it is a party continue in full force and
effect notwithstanding the execution and delivery of this Amendment or the
consummation of the transactions contemplated hereby. There are no defenses,
offsets or counterclaims to the performance by any of the undersigned Loan
Parties of their respective indebtedness, liabilities and obligations under the
Loan Documents.

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 7
<PAGE>

      10. Reference to Credit Agreement. This Amendment shall constitute a Loan
Document. Each of the Loan Documents, including the Credit Agreement, the
Amendment Documents and any and all other agreements, documents or instruments
now or hereafter executed and/or delivered pursuant to the terms hereof or
pursuant to the terms of the Credit Agreement as amended hereby, are (if and to
the extent necessary) hereby amended so that any reference in such Loan
Documents to the Credit Agreement shall mean a reference to the Credit Agreement
as amended hereby.

      11. Severability. Any provision of this Amendment held by a court of
competent jurisdiction to be invalid or unenforceable shall not impair or
invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

      12. Successors and Assigns. This Amendment is binding upon and shall inure
to the benefit of the Administrative Agent, the Lenders, the Borrower and the
other Loan Parties and their respective successors and assigns; provided,
however, that neither the Borrower nor any of the other Loan Parties may assign
or transfer any of its rights or obligations hereunder without the prior written
consent of the Administrative Agent and the Lenders.

      13. Headings. The headings, captions and arrangements used in this
Amendment are for convenience only and shall not affect the interpretation of
this Amendment.

      14. Certain Resolutions. The Borrower shall, on or before March 3, 2000,
deliver, or cause to be delivered, to the Administrative Agent resolutions of
the Board of Directors of each of the Loan Parties, other than the Borrower,
certified by the Secretary or an Assistant Secretary of such Loan Party which
authorized the execution, delivery and performance by such Loan Party of this
Amendment and the other Amendment Documents to which such Loan Party is or is to
be a party.

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 8
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
executed and delivered by their duly authorized officers effective as of the day
and year first above written.

                                          BORROWER:

                                          NET-TEL CORPORATION

                                          By:    /s/ Craig Bandes
                                                 ____________________________
                                          Name:  Craig Bandes
                                                 ____________________________
                                          Title: Senior Vice President,
                                                 Corporate Development

                                          HOLDINGS:

                                          NET-TEL COMMUNICATIONS, INC.

                                          By:    /s/ Craig Bandes
                                                 ____________________________
                                          Name:  Craig Bandes
                                                 ____________________________
                                          Title: Senior Vice President,
                                                 Corporate Development

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 9
<PAGE>

                                          ADMINISTRATIVE AGENT:

                                          NORTEL NETWORKS INC.,
                                          as Administrative Agent

                                          By:    /s/ Mitchell Stone
                                                 ____________________________
                                          Name:  Mitchell Stone
                                                 ____________________________
                                          Title: Director
                                                 ____________________________

                                          LENDERS:

                                          NORTEL NETWORKS INC.

                                          By:    /s/ Mitchell Stone
                                                 ____________________________
                                          Name:  Mitchell Stone
                                                 ____________________________
                                          Title: Director
                                                 ____________________________

                                          ALLIED CAPITAL CORPORATION

                                          By:    /s/ Thomas Aiken
                                                 ____________________________
                                          Name:  Thomas Aiken
                                                 ____________________________
                                          Title: Associate
                                                 ____________________________

FIRST AMENDMENT TO CREDIT AGREEMENT - Page 10<PAGE>

                                                                  Exhibit 10.8.2

[*] IMPORTANT NOTE:  Certain material, indicated by an asterisk ("*"), has been
omitted from this document pursuant to a request for confidential treatment. The
omitted material has been filed separately with the Securities and Exchange
Commission.

                     SECOND AMENDMENT TO CREDIT AGREEMENT
                     ------------------------------------

     This SECOND AMENDMENT TO CREDIT AGREEMENT (this "Amendment") is dated
                                                      ---------
effective as of December 31, 1999 (the "Amendment Date") unless otherwise
                                        --------------
expressly stated herein, is by and among NET-TEL CORPORATION (the "Borrower"), a
                                                                   --------
Florida corporation, NET-TEL COMMUNICATIONS, INC. ("Holdings"), a Delaware
                                                    --------
corporation, NET-TEL CORPORATION OF VIRGINIA, INC., a Virginia corporation

("NET-tel Virginia"), each of the undersigned Lenders (as hereinafter defined)
------------------
and NORTEL NETWORKS INC., a Delaware corporation, as administrative agent for
itself and the other Lenders (in such capacity, together with its successors in
such capacity, the "Administrative Agent").
                    --------------------

                                   RECITALS:
                                   --------

     A.  Pursuant to that certain Credit Agreement dated as of July 28, 1999 (as
amended by that certain First Amendment to Credit Agreement dated effective as
of July 28, 1999 and as the same may be further amended, modified, renewed,
extended, restated or supplemented from time to time, the "Credit Agreement") by
                                                           ----------------
and among the Borrower, each of the lending entities which is a party thereto or
which may from time to time become a party thereto as a lender or any successor
or assignee thereof (individually, a "Lender" and, collectively, the "Lenders")
                                      ------                          -------
and the Administrative Agent, the Lenders agreed to provide, with the assistance
of the Administrative Agent, (i) a $120,000,000 advancing term loan facility to
finance a portion of the Borrower's costs to purchase Nortel Networks Goods and
Services (as defined in the Credit Agreement) and to finance certain Eligible
Third-Party Expenses (as defined in the Credit Agreement), (ii) a $10,000,000
term loan facility to finance certain of the Borrower's working capital needs
and for general corporate purposes, and (iii) a $10,000,000 revolving loan
facility to finance certain of the Borrower's working capital needs and for
general corporate purposes and in the ordinary course of business.

     B.  The Borrower, the Administrative Agent and the Lenders desire to amend
the Credit Agreement upon the terms and conditions contained herein.

                                  AGREEMENTS:
                                  ----------

     NOW, THEREFORE, in consideration of the premises and mutual covenants and
agreements contained herein and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
hereby agree as follows:

     1.  Terms Defined.  Unless otherwise defined or stated in this Amendment,
         -------------
each capitalized term used in this Amendment has the meaning given to such term
in the Credit Agreement (as amended by this Amendment).

     2.  Amendments.
         ----------

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 1
<PAGE>

     (a) The definition of the term "Average Customer Count" set forth in

Section 1 of the Credit Agreement is hereby amended and restated to read in its
---------
entirety as follows:

         "Average Customer Count" means, as of the last day of any calendar
          ----------------------
     quarter, the quotient obtained by dividing (a) the sum of the number of
     customers of the Borrower and its Consolidated Subsidiaries as of the last
     day of each of the three months during such calendar quarter, each of which
     customers has average billings in excess of $100 per month, by (b) three.

     (b) The definition of the term "Average Monthly Gross Revenues Per
Customer" set forth in Section 1 of the Credit Agreement is hereby amended and
                       ---------
restated to read in its entirety as follows:

         "Average Monthly Gross Revenues Per Customer" means, as to the
          -------------------------------------------
     Borrower and its Consolidated Subsidiaries and with respect to any calendar
     quarter, the quotient obtained by dividing (a) the remainder of (i) the
     Average Gross Revenues for such calendar quarter minus (ii) the Average
                                                      -----
     Gross Revenues for such calendar quarter attributable to customers having
     average billings (per customer) of less than $100 per month during such
     calendar quarter by (b) the Average Customer Count as of the last day of
     such calender quarter.

     (c) Subclause (g)(iv)  of the definition of the term "Permitted Liens"
         -----------------
contained in Section 1.1 of the Credit Agreement is hereby amended and restated
             -----------
to read in its entirety as follows:

         (iv) the Debt secured by all such Liens, when aggregated with the Debt
     secured by all purchase-money Liens at any time outstanding (whenever
     incurred or created) and all Liens in connection with any conditional sale
     or other title retention agreement or Capital Lease Obligation existing as
     of the Closing Date or at any other time, shall not exceed (A) $*
     outstanding in the aggregate at any time on or before December 30, 2001,
     (B) $* outstanding in the aggregate on December 31, 2001 or at any time
     thereafter on or before December 30, 2002, (C) $* outstanding in the
     aggregate on December 31, 2002 or at any time thereafter on or before
     December 30, 2003 and (D) $* outstanding in the aggregate on December 31,
     2003 or at any time thereafter.

     (d) Effective as of April 21, 2000, the definition of the term "Term Loans
A Commitment" set forth in Section 1 of the Credit Agreement is hereby amended
                           ---------
and restated to read in its entirety as follows:

         "Term Loans A Commitment" means, as to any Lender, the obligation (if
          -----------------------
     any) of such Lender to make or continue Term Loans A hereunder in an
     aggregate principal amount up to but not exceeding the amount set forth
     opposite the name of such Lender on the signature pages hereto under the
     heading "Term Loans A Commitment" or, if such Lender is a party to an
     Assignment and Acceptance, the amount of the "Term Loans A Commitment" set
     forth in the most recent Assignment and Acceptance of such Lender, as the
     same may be reduced or terminated pursuant

*Confidential treatment requested. The redacted material has been separately
 filed with the Securities and Exchange Commission.

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 2
<PAGE>

     to Section 2.13 or 11.2, and "Term Loans A Commitments" means such
        ------------    ----       ------------------------
     obligations of all Lenders. As of March 27, 2000, the aggregate principal
     amount of the Term Loans A Commitments is the remainder of (a) $115,000,000
     minus (b) the aggregate principal amount of the Term Loans A advanced on or
     before such date.

     (e) Section 8.1(c) of the Credit Agreement is hereby amended and restated
         --------------
to read in its entirety as follows:

         (c) Compliance Certificate.  Concurrently with the delivery of each of
             ----------------------
     the financial statements referred to in Sections 8.1(a) and 8.1(b) and
                                             ---------------     ------
     within 45 days after the end of the fourth quarter of each fiscal year of
     the Borrower, a Compliance Certificate of a Responsible Officer of the
     Borrower substantially in the form of Exhibit D hereto, appropriately
                                           ---------
     completed, stating that, to the best of such officer's knowledge, no
     Default has occurred and is continuing or, if a Default has occurred and is
     continuing, stating the nature thereof and the action that has been taken
     and is proposed to be taken with respect thereto;

     (f) Section 8.1(p) of the Credit Agreement is hereby amended and restated
         --------------
to read in its entirety as follows:

         (p) Accounts Receivable and Payable.  As soon as available and in any
             -------------------------------
     event within (i) 45 days after the end of each of the first three quarters
     of each fiscal year of the Borrower and (ii) 90 days after the end of the
     fourth quarter of each fiscal year of the Borrower, an aged trial balance
     of all then-existing Receivables and all then existing accounts payable of
     the Borrower and its Subsidiaries;

     (g) Section 8.1(r) of the Credit Agreement is hereby amended and restated
         --------------
to read in its entirety as follows:

         (r) Quarterly Report as to Key Business Statistics.  As soon as
             ----------------------------------------------
     available and in any event within (i) 45 days after the end of each of the
     first three quarters of each fiscal year of the Borrower and (ii) 90 days
     after the end of the fourth quarter of each fiscal year of the Borrower,
     reports as to key business and operational statistics of the Borrower and
     its Subsidiaries, including, without limitation, reports as to the number
     of customers, number of Sales Representatives and such related information
     as the Administrative Agent may reasonably request from time to time;

     (h) Section 8.1(s) of the Credit Agreement is hereby amended and restated
         --------------
to read in its entirety as follows:

         (s) Borrowing Base Reports and Agings.  As soon as available and in
             ---------------------------------
     any event within (i) 45 days after the end of each of the first three
     quarters of each fiscal year of the Borrower and (ii) 90 days after the end
     of the fourth quarter of each fiscal year of the Borrower and, in any event
     concurrently with the making of each Revolving Loan hereunder and from time
     to time upon the request of the Administrative Agent after the occurrence
     of a Default, (A) a Borrowing Base Report

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 3
<PAGE>

     duly completed and (B) an aged trial balance of all then-existing
     Receivables and all then existing accounts payable of the Borrower and its
     Subsidiaries; and

     (i) Section 10.13 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as follows:

          Section 10.13  Operating Leases.  The Borrower will not, and will not
                         ----------------
     permit any Subsidiary of the Borrower to, at any time enter into or be a
     party to operating leases that in the aggregate obligate the Borrower
     and/or its Subsidiaries to make aggregate payments during any fiscal year
     in excess of (a) for all operating leases other than operating leases of
     real Property entered into in the ordinary course of the Borrower's and/or
     its Subsidiaries' business, $* and (b) for all operating leases of real
     Property, the lesser of (i) *% of the Gross Revenues of the Borrower and
     its Consolidated Subsidiaries for such fiscal year or (ii) $*.

     (j) Section 12.2 of the Credit Agreement is hereby amended by amending and
         ------------
restating the last sentence thereof which shall read in its entirety as follows:

     "Without limiting the generality of the foregoing, (a) Nortel Networks has
     purchased certain equity securities issued by Holdings and (b) it is
     contemplated that Nortel Networks and/or certain other Lenders will be the
     holder of warrants issued by Holdings."

     (k) Effective as of April 21, 2000, the amount of "120,000,000" set forth
next to "Term Loans A Commitment" on the signature page for Nortel Networks as a
Lender is hereby amended to read "115,000,000".

     (l) The "Address for Notices" set forth under the signature of each of the
Administrative Agent and Nortel Networks as a Lender is hereby amended and
restated to read in its entirety as follows:

          Nortel Networks Inc.
          Mail Stop 991 15 A40
          2221 Lakeside Blvd.
          Richardson, Texas  75082-4399
          Attention:  Mitchell D. Stone
                      Director, Customer Finance
          Telephone:  972-684-0395
          Telecopy:   972-684-3679

          and

          Nortel Networks Inc.
          Mail Stop 468/05/B40
          2100 Lakeside Blvd.
          Richardson, Texas 75082-4399

* Confidential treatment requested.  The redacted material has been separately
  filed with the Securities and Exchange Commission.

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 4
<PAGE>

          Attention:  Kimberly Poe, Director
                      Loan Administration
          Telephone:  972-684-7687
          Telecopy:   972-685-3613

     (m) Schedule 10.2 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 1 to this
                                     ---------------------------
Amendment.

     (n) Schedule 10.3 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 2 to this
                                     ---------------------------
Amendment.

     (o) Schedule 10.4 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 3 to this
                                     ---------------------------
Amendment.

     (p) Schedule 10.5 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 4 to this
                                     ---------------------------
Amendment.

     (q) Schedule 10.6 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 5 to this
                                     ---------------------------
Amendment.

     (r) Schedule 10.7 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 6 to this
                                     ---------------------------
Amendment.

     (s) Schedule 10.8 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 7 to this
                                     ---------------------------
Amendment.

     (t) Schedule 10.9 of the Credit Agreement is hereby amended and restated to
         -------------
read in its entirety as set forth on Second Amendment Schedule 8 to this
                                     ---------------------------
Amendment.

     (u) Schedule 10.10 of the Credit Agreement is hereby amended and restated
         --------------
to read in its entirety as set forth on Second Amendment Schedule 9 to this
                                        ---------------------------
Amendment.

     (v) Schedule 10.11 of the Credit Agreement is hereby amended and restated
         --------------
to read in its entirety as set forth on Second Amendment Schedule 10 to this
                                        ----------------------------
Amendment.

     (w) Schedule 10.12 of the Credit Agreement is hereby amended and restated
         --------------
to read in its entirety as set forth on Second Amendment Schedule 11 to this
                                        ----------------------------
Amendment.

     3.  Conditions Precedent.  The effectiveness of this Amendment is subject
         --------------------
to the satisfaction of each of the following conditions precedent, all of which
conditions precedent must be satisfied on or before April 30, 2000 (unless any
one or more of such conditions precedent are waived in writing by the
Administrative Agent):

     (a) The Administrative Agent shall have received all of the following, each
dated (where applicable and unless otherwise indicated) the date of this
Amendment, in form and substance satisfactory to the Administrative Agent:

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 5
<PAGE>

          (i)    Amendment.  This Amendment as executed by the parties hereto;
                 ---------

          (ii)   Resolutions.  Resolutions of the Board of Directors of each
                 -----------
     Loan Party certified by its Secretary or an Assistant Secretary which
     authorize the execution, delivery and performance by such Loan Party of
     this Amendment; and

          (iii)  Fees, Costs and Expenses.  If so requested by the
                 ------------------------
     Administrative Agent, all fees, costs and expenses (including, without
     limitation, attorneys' fees and expenses) incurred by the Administrative
     Agent and each of the Lenders incident to this Amendment or required to be
     paid in accordance with Section 13.1 of the Credit Agreement, to the extent
                             ------------
     incurred and submitted to the Borrower, shall have been paid in full by the
     Borrower;

     (b) The representations and warranties contained herein and in all other
Loan Documents, as amended hereby, shall be true and correct as of the date
hereof as if made again on and as of the date hereof (except if and to the
extent that such representations and warranties are or were expressly made only
as of another specific date); and

     (c) No Event of Default shall have occurred and be continuing (immediately
after giving effect to this Amendment).

     Upon the satisfaction of all conditions precedent set forth in this

Paragraph 3 and upon request of confirmation of such satisfaction made by the
-----------
Borrower to the Administrative Agent, the Administrative Agent shall confirm
such satisfaction in writing to the Borrower.

     4.  Governing Law.  THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS SHALL BE
         -------------
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK
(WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES) AND APPLICABLE LAWS OF THE U.S.

     5.  Counterparts.  This Amendment may be executed in any number of
         ------------
counterparts, all of which when taken together shall constitute one agreement,
and any of the parties hereto may execute this Amendment by signing any such
counterpart.

     6.  No Oral Agreements.  THIS AMENDMENT, TOGETHER WITH THE CREDIT AGREEMENT
         ------------------
AND THE OTHER LOAN DOCUMENTS AS WRITTEN, REPRESENT THE FINAL AGREEMENTS BETWEEN
AND AMONG THE PARTIES HERETO RELATING TO THE SUBJECT MATTER HEREOF AND MAY NOT
BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL
AGREEMENTS OF THE PARTIES.  THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN (A)
THE BORROWER OR ANY OTHER LOAN PARTY AND (B) THE ADMINISTRATIVE AGENT AND/OR ANY
LENDER.

     7.  Agreement Remains in Effect; No Waiver.  Except as expressly provided
         --------------------------------------
herein, all terms and provisions of the Credit Agreement and the other the Loan
Documents shall remain unchanged and in full force and effect and are hereby
ratified and confirmed.  No waiver by the Administrative Agent or any Lender of
any Default or Event of Default shall be deemed to be a

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 6
<PAGE>

waiver of any other Default or Event of Default. No delay or omission by the
Administrative Agent or any Lender in exercising any power, right or remedy
shall impair such power, right or remedy or be construed as a waiver thereof or
an acquiescence therein, and no single or partial exercise of any such power,
right or remedy shall preclude other or further exercise thereof or the exercise
of any other power, right or remedy under the Credit Agreement, the Loan
Documents or otherwise.

     8.  Representations and Warranties.  All representations and warranties
         ------------------------------
made in this Amendment or any other Loan Document shall survive the execution
and delivery of this Amendment and the other Loan Documents, and no
investigation by the Administrative Agent or any closing shall affect the
representations and warranties or the right of the Administrative Agent and the
Lenders to rely upon such representations and warranties.

     9.  Ratification of Obligations under the Loan Documents.  Each of the
         ----------------------------------------------------
undersigned Loan Parties acknowledges and consents to the terms of this
Amendment and agrees that all of its indebtedness, liabilities and obligations
under the Loan Documents to which it is a party continue in full force and
effect notwithstanding the execution and delivery of this Amendment or the
consummation of the transactions contemplated hereby. There are no defenses,
offsets or counterclaims to the performance by any of the undersigned Loan
Parties of their respective indebtedness, liabilities and obligations under the
Loan Documents.

     10.  Reference to Credit Agreement.  This Amendment shall constitute a Loan
          -----------------------------
Document.  Each of the Loan Documents, including the Credit Agreement and any
and all other agreements, documents or instruments now or hereafter executed
and/or delivered pursuant to the terms hereof or pursuant to the terms of the
Credit Agreement as amended hereby, are (if and to the extent necessary) hereby
amended so that any reference in such Loan Documents to the Credit Agreement
shall mean a reference to the Credit Agreement as amended hereby.

     11.  Severability.  Any provision of this Amendment held by a court of
          ------------
competent jurisdiction to be invalid or unenforceable shall not impair or
invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

     12.  Successors and Assigns.  This Amendment is binding upon and shall
          ----------------------
inure to the benefit of the Administrative Agent, the Lenders, the Borrower and
the other Loan Parties and their respective successors and assigns; provided,
                                                                    --------
however, that neither the Borrower nor any of the other Loan Parties may assign
-------
or transfer any of its rights or obligations hereunder without the prior written
consent of the Administrative Agent and the Lenders.

     13.  Term Loans A Commitments.  The Administrative Agent, the Lenders and
          ------------------------
the Borrower agree that, as of April 21, 2000, the aggregate principal amount of
the Term Loans A Commitments is the remainder of (a) $115,000,000 minus (b) the
                                                                  -----
aggregate principal amount of the Term Loans A advanced on or before April 21,
2000.

     14.  Headings.  The headings, captions and arrangements used in this
          --------
Amendment are for convenience only and shall not affect the interpretation of
this Amendment.

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 7
<PAGE>

     15.  Fees, Costs and Expenses.  The Borrower hereby agrees, on demand, to
          ------------------------
pay or reimburse the Administrative Agent and each of the Lenders for paying all
reasonable out-of-pocket costs and expenses of the Administrative Agent accrued
in connection with the drafting, preparation, negotiation, execution and
delivery of this Amendment and all Loan Documents to be entered into in
connection herewith, including, without limitation, the reasonable fees and
expenses of legal counsel (including all local counsel) for the Administrative
Agent.

              [Remainder of this page intentionally left blank.]

SECOND AMENDMENT TO CREDIT AGREEMENT - Page 8
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
executed and delivered by their duly authorized officers effective as of the day
and year first above written.

                                    BORROWER:
                                    --------

                                    NET-TEL CORPORATION

                                    By:    /s/ James F. Kenefick
                                       ---------------------------
                                    Name:      James F. Kenefick
                                         -------------------------
                                    Title:     CEO
                                          ------------------------

                                    HOLDINGS:
                                    --------

                                    NET-TEL COMMUNICATIONS, INC.

                                    By:    /s/ James F. Kenefick
                                       ---------------------------
                                    Name:      James F. Kenefick
                                         -------------------------
                                    Title:     CEO
                                          ------------------------

                                    NET-TEL VIRGINIA:
                                    ----------------

                                    NET-TEL CORPORATION OF VIRGINIA, INC.

                                    By:    /s/ James F. Kenefick
                                       ---------------------------
                                    Name:      James F. Kenefick
                                         -------------------------
                                    Title:     CEO
                                          ------------------------

SECOND AMENDMENT TO CREDIT AGREEMENT
<PAGE>

                                    ADMINISTRATIVE AGENT:
                                    --------------------

                                    NORTEL NETWORKS INC.,
                                    as Administrative Agent

                                    By:    /s/ Mitchell L. Stone
                                       ---------------------------
                                    Name:      Mitchell L. Stone
                                         -------------------------
                                    Title:     Director
                                          ------------------------

                                    LENDERS:
                                    -------

                                    NORTEL NETWORKS INC.

                                    By:    /s/ Mitchell L. Stone
                                       ---------------------------
                                    Name:      Mitchell L. Stone
                                         -------------------------
                                    Title:     Director
                                          ------------------------

                                    ALLIED CAPITAL CORPORATION

                                    By:    /s/ Scott S. Binder
                                       ---------------------------
                                    Name:      Scott S. Binder
                                         -------------------------
                                    Title:     Principal
                                          ------------------------

SECOND AMENDMENT TO CREDIT AGREEMENT

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