<PAGE>

           THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT
              OF 1933 AND MAY NOT BE SOLD, TRANSFERRED OR OTHERWISE
                  DISPOSED OF UNLESS REGISTERED UNDER THAT ACT
                 OR AN EXEMPTION FROM REGISTRATION IS AVAILABLE.

                                 IFX CORPORATION

                           CONVERTIBLE PROMISSORY NOTE

$950,000.00           Dated: October 11, 2001

          FOR VALUE RECEIVED IFX CORPORATION, a Delaware corporation (the
"Company"), hereby promises to pay to UBS CAPITAL AMERICAS III, L.P. (the
 -------
"Payee"), or its registered assigns, the principal amount of Nine Hundred Fifty
 -----
Thousand and 00/100 Dollars ($950,000.00) together with interest thereon
calculated from the date hereof in accordance with the provisions of this
Convertible Promissory Note.

          Certain capitalized terms are defined in Section 9 hereof.
                                                   ---------

          1.  Payment of Interest. Interest shall accrue at a rate equal to ten
              -------------------
percent (10%) per annum (the "Interest Rate") on the unpaid principal amount of
                              -------------
this Convertible Note outstanding from time to time; provided that so long as
                                                     --------
any Event of Default has occurred and is continuing, interest shall accrue to
the extent permitted by law at the rate of the Interest Rate plus two percent
(2%) per annum on the unpaid principal amount of this Convertible Note
outstanding from time to time for the period beginning on the date on which such
Event of Default occurs and ending on the date on which such Event of Default
ceases to exist. Interest shall be computed on the basis of the actual number of
days elapsed and a 360-day year.

          2.  Maturity Date. Notwithstanding Section 3 below, the entire
              -------------
principal amount of this Convertible Note and all accrued but unpaid interest
thereon shall be due and payable in full in cash in immediately available funds
on December 11, 2001 (the "Maturity Date"). Any overdue principal and overdue
                           -------------
interest together with any interest thereon, shall be due and payable upon
demand.

          3.  Conversion. (i) All of the principal amount of this Convertible
              ----------
Note plus accrued and unpaid interest thereon shall be converted into shares of
the Series C Preferred Stock at a conversion price set forth in the Series C
Stock Purchase Agreement, upon consummation of the Series C Financing. The
Series C Preferred Stock to be issued upon any such conversion shall have the
same rights, preferences and privileges as the shares of the Series C Preferred
Stock issued in the Series C Financing. The Payee, upon making such conversion,
shall be entitled to all the benefits of any agreements entered into among the
Company and the holders of the Series C Preferred Stock. No fractional shares
shall be issued upon a conversion into Series C Preferred Stock. In lieu of any
fractional shares to which Payee would otherwise be entitled, the Company shall
pay cash equal to such fraction multiplied by $3.00.

<PAGE>

          (ii)  The conversion of this Convertible Note shall be deemed to have
been effected as of the close of business on the date of the consummation of the
Series C Financing. At such time as such conversion has been effected, the
rights of the holder of this Convertible Note as the holder of such note shall
cease, and the Person or Persons in whose name or names any certificate or
certificates for shares of the Series C Preferred Stock are to be issued upon
such conversion shall be deemed to have become the holder or holders of record
of the shares of such Series C Preferred Stock represented thereby.

          (iii) At the closing of the Series C Financing, the Company shall
deliver to the converting holder a certificate or certificates representing the
number of shares of Series C Preferred Stock issuable by reason of such
conversion in such name or names and such denomination or denominations as the
converting holder has specified.

          (iv)  The issuance of certificates for shares of Series C Preferred
Stock upon conversion of this Convertible Note shall be made without charge to
the holder hereof for any issuance tax in respect thereof or other cost incurred
by the Company in connection with such conversion and the related issuance of
shares of Series C Preferred Stock.

          (v)   The Company shall not close its books against the transfer of
Series C Preferred Stock issued or issuable upon conversion of this Convertible
Note in any manner which interferes with the timely conversion of this
Convertible Note. The Company shall assist and cooperate with any holder of this
Convertible Note required to make any governmental filings or obtain any
governmental approval prior to or in connection with the conversion of this
Convertible Note (including, without limitation, making any filings required to
be made by the Company).

          (vi)  The Company shall at all times reserve and keep available out of
its authorized but unissued shares of Series C Preferred Stock, solely for the
purpose of issuance upon conversion hereunder, such number of shares of Series C
Preferred Stock issuable upon conversion. All shares of such capital stock which
are so issuable shall, when issued, be duly and validly issued, fully paid and
nonassessable and free from all taxes, liens and charges. The Company shall take
all such actions as may be necessary to assure that all such shares of capital
stock may be so issued without violation of any applicable law or governmental
regulation or any requirements of any domestic securities exchange upon which
such shares of capital stock may be listed.

          4.    Method of Payments.

          (a)   Payment. Notwithstanding anything contained elsewhere in this
                -------
Convertible Note to the contrary, the Company will pay all sums for principal,
interest, premiums, dividends or otherwise becoming due on this Convertible Note
not later than 1:00 p.m. New York time, on the Maturity Date, in immediately
available funds, in accordance with the payment instructions that the Payee may
designate in writing, without the presentation or surrender of such Convertible
Note or the making of any notation thereon. Any payment made after 1:00 p.m. New
York time, on a Business Day will be deemed made on the next following Business
Day. If the Maturity Date falls on a day that is not a Business Day, such due
date shall be extended to the next succeeding Business Day, and interest shall
be payable on any

                                       -2-

<PAGE>

principal so extended for the period of such extension. All amounts payable
under this Convertible Note shall be paid free and clear of, and without
reduction by reason of, any deduction, set-off or counterclaim. The Company will
afford the benefits of this Section to the Payee and to each other Person
holding this Convertible Note.

          (b)  Transfer and Exchange. Upon surrender of any Convertible Note for
               ---------------------
registration of transfer or for exchange to the Company at its principal office,
the Company at its sole expense will execute and deliver in exchange therefor a
new Convertible Note or Convertible Notes, as the case may be, as requested by
the holder or transferee, which aggregate the unpaid principal amount of such
Convertible Note, registered as such holder or transferee may request, dated so
that there will be no loss of interest on the Convertible Note and otherwise of
like tenor. The issuance of new Convertible Notes shall be made without charge
to the holder(s) of the surrendered Convertible Note for any issuance tax in
respect thereof or other cost incurred by the Company in connection with such
issuance, provided that each Convertible Noteholder shall pay any transfer taxes
associated therewith. The Company shall be entitled to regard the registered
holder of this Convertible Note as the holder of the Convertible Note so
registered for all purposes until the Company or its agent, as applicable, is
required to record a transfer of this Convertible Note on its register.

          (c)  Replacement. Upon receipt of evidence reasonably satisfactory to
               -----------
the Company of the loss, theft, destruction or mutilation of any Convertible
Note and, in the case of any such loss, theft or destruction of any Convertible
Note, upon receipt of an indemnity reasonably satisfactory to the Company or, in
the case of any such mutilation, upon the surrender and cancellation of such
Convertible Note, the Company, at its expense, will execute and deliver, in lieu
thereof, a new Convertible Note of like tenor and dated the date of such lost,
stolen, destroyed or mutilated Convertible Note.

          5.   Representations and Warranties of Payee. Payee represents and
               ---------------------------------------
warrants to Company that Payee is acquiring this Convertible Note and any and
all shares of capital stock into which this Convertible Note is convertible
solely for its own account for the purpose of investment and not with a view to
or for sale in connection with any distribution thereof, and has no present
intention or plan to effect any distribution thereof. The shares of capital
stock issuable upon conversion hereof will bear a legend to the following
effect:

                         "The securities represented by this certificate have
                         not been registered under the Securities Act of 1933,
                         as amended (the "Act"), or the laws of any state and
                                          ---
                         may not be sold or transferred except in compliance
                         with the Act and such laws."

          6.   Representations and Warranties of the Company. The Company
               ---------------------------------------------
represents and warrants to Payee that:

          (a)  Organization and Qualification. The Company and each of its
               ------------------------------
subsidiaries is an entity duly organized, validly existing and in good standing
under the laws of its jurisdiction of organization, with power and authority to
conduct its business as it is now being

                                       -3-

<PAGE>

conducted, to own or use its properties and assets that it purports to own or
use and, in the case of the Company, to perform its obligations under this
Convertible Note. The Company and each of its subsidiaries is duly qualified to
do business as a foreign company and is in good standing under the laws of each
state or other jurisdiction in which either the ownership or use of the
properties owned or used by it, or the nature of the activities conducted by it,
requires such qualification.

          (b)  Absence of Conflicts. Neither the execution, delivery and
               --------------------
performance of this Convertible Note by the Company, nor the consummation of the
transactions contemplated hereby, nor compliance by the Company with any of the
provisions hereof, will (a) violate, conflict with, or result in a breach of any
provision of, constitute a default under, or permit or result in the termination
of, acceleration of any obligation under, or creation of a lien under any of the
terms, conditions or provisions of, (i) the certificate of incorporation, bylaws
or stockholders agreements of the Company, or (ii) any note, mortgage,
indenture, contract, agreement or license by which the Company or any of the
properties or assets thereof may be bound, or to which the Company or any
subsidiary thereof or any of the properties or assets thereof may be subject, or
(b) violate or conflict with any law, rule, regulation, judgment, ruling, order,
writ, injunction or decree applicable to the Company or any subsidiary thereof
or any of the properties or assets thereof.

          (c)  Authorization of Agreements, Etc. Each of (i) the execution and
               --------------------------------
delivery by the Company of this Convertible Note, (ii) the performance by the
Company of its obligations hereunder, and (iii) the issuance, sale and delivery
by the Company of this Convertible Note and the shares of Series C Preferred
Stock issuable upon conversion thereof has been duly authorized by all necessary
corporate action of the Company.

          (d)  Validity. This Convertible Note has been duly executed and
               --------
delivered by the Company and constitutes the legal, valid and binding obligation
of the Company, enforceable against the Company in accordance with its terms.

          7.   Conditions Precedent. The obligation of the Payee to make the
               --------------------
loan hereunder is subject to the fulfillment of the following condition
precedent: the Company, the Payee and other purchasers signatory thereto shall
have entered into the Series C Stock Purchase Agreement.

          8.   Events of Default. If any of the following events takes place
               -----------------
before the Maturity Date (each, an "Event of Default"), Payee at its option may
                                    ----------------
declare all principal and accrued and unpaid interest thereon and all other
amounts payable under this Convertible Note immediately due and payable;
provided, however, that this Convertible Note shall automatically become due and
--------  -------
payable without any declaration in the case of an Event of Default specified in
clause 3 or 5, below:

          (1)  The Company fails to make payment of the full amount due under
               this Convertible Note on demand at the Maturity Date; or

          (2)  A receiver, liquidator or trustee is appointed by a court order
               (i) of the Company or (ii) for any substantial part of the
               Company's assets or properties; or

                                       -4-

<PAGE>

          (3)  The Company is adjudicated bankrupt or insolvent; or
          (4)  Any of the Company's property is sequestered by or in consequence
               of a court order and such order remains in effect for more than
               30 days; or
          (5)  The Company files a petition in voluntary bankruptcy or requests
               reorganization under any provision of any bankruptcy,
               reorganization or insolvency law or consents to the filing of any
               petition against it under such law, or
          (6)  Any petition against the Company is filed under bankruptcy,
               receivership or insolvency law; or
          (7)  The Company makes a formal or informal general assignment for the
               benefit of its creditors, or admits in writing its inability to
               pay debts generally when they become due, or consents to the
               appointment of a receiver, liquidator or trustee of the Company
               or for all or any part of its property; or
          (8)  An attachment or execution is levied against any substantial part
               of the Company's assets that is not released within 30 days; or
          (9)  The Company dissolves, liquidates or ceases business activity, or
               transfers any major portion of its assets other than in the
               ordinary course of business; or
          (10) The Company breaches any covenant or agreement on its part
               contained in this Convertible Note;
          (11) There exists any material inaccuracy or untruthfulness of any
               representation or warranty of the Company set forth in this
               Convertible Note; or
          (12) The Company shall default under any promissory note, credit
               agreement, loan agreement, conditional sales contract, guarantee,
               lease, indenture, bond, debenture or other material obligation to
               which it is a party whatsoever having an aggregate outstanding
               amount greater than $500,000 and a party thereto or a holder
               thereof is entitled to accelerate the obligations of the Company
               (excluding for purposes hereof any current default under the
               Company's existing indebtedness to Nortel Networks (CALA), Inc.).

          9.   Definitions.
               -----------

          "Business Day" means a day (other than a Saturday or Sunday) on which
           ------------
banks generally are open in New York, New York for the conduct of substantially
all of their activities.

          "Convertible Note" shall mean this Convertible Promissory Note,
           ----------------
together with any other convertible notes in the form hereof issued upon
transfer or exchange hereof, in whole or in part.

          "Convertible Noteholder" with respect to any Convertible Note, means
           ----------------------
at any time each Person then the record owner of such Convertible Note and
"Convertible Noteholders" means all of such Convertible Noteholders
collectively.

                                       -5-

<PAGE>

          "Person" means any person or entity of any nature whatsoever,
           ------
specifically including an individual, a firm, a company, a corporation, a
partnership, a limited liability company, a trust or other entity.

          "Series C Preferred Stock" means the new series of convertible
           ------------------------
preferred stock of the Company issued in a Series C Financing.

          "Series C Financing" means the issuance of a new series of convertible
           ------------------
preferred stock of the Company to one or more investors for cash following the
date of issuance of this Convertible Note on terms and conditions set forth in
the Series C Stock Purchase Agreement.

          "Series C Stock Purchase Agreement" means the IFX Corporation Series C
           ---------------------------------
Convertible Preferred Stock Purchase Agreement dated as of October 11, 2001
among the Company, UBS Capital Americas III, L.P. and the other parties named
therein.

          10.  Expenses of Enforcement, etc. The Company agrees to pay all fees
               ----------------------------
and expenses incurred by the Payee in connection with the negotiation, execution
and delivery of this Convertible Note. The Company agrees to pay all reasonable
fees and expenses incurred by the Payee in connection with any amendments,
modifications, waivers, extensions, renewals, renegotiations or "workouts" of
the provisions hereof or incurred by the Payee in connection with the
enforcement or protection of the Payee's rights in connection with this
Convertible Note, or in connection with any pending or threatened action,
proceeding, or investigation relating to the foregoing, including but not
limited to the reasonable fees and expenses of counsel for the Payee. The
Company indemnifies the Payee and its directors, managers, affiliates, partners,
members, officers, employees and agents against, and agrees to hold the Payee
and each such person and/or entity harmless from, any and all losses, claims,
damages, liabilities and related expenses, including reasonable counsel fees and
expenses, incurred by or asserted against the Payee or any such person or entity
arising out of, in any way connected with, or as a result of (i) the
consummation of the loan evidenced by this Convertible Note and the use of the
proceeds thereof or (ii) any claim, litigation, investigation or proceedings
relating to any of the foregoing, whether or not the Payee or any such person or
entity is a party thereto.

          11.  Amendment and Waiver. The provisions of this Convertible Note may
               --------------------
not be modified, amended or waived, and the Company may not take any action
herein prohibited, or omit to perform any act herein required to be performed by
it without the written consent of the Payee.

          12.  Remedies Cumulative. No remedy herein conferred upon the Payee is
               -------------------
intended to be exclusive of any other remedy and each and every such remedy
shall be cumulative and shall be in addition to every other remedy given
hereunder or now or hereafter existing at law or in equity or by statute or
otherwise.

          13.  Remedies Not Waived. No course of dealing between the Company and
               -------------------
the Payee or any delay on the part of the Payee in exercising any rights
hereunder shall operate as a waiver of any right of the Payee.

                                       -6-

<PAGE>

          14.  Assignments. The Payee may assign, participate, transfer or
               -----------
otherwise convey this Convertible Note and any of its rights or obligations
hereunder or interest herein to any Person, and this Convertible Note shall
inure to the benefit of the Payee's successors and assigns. The Company shall
not assign or delegate this Convertible Note or any of its liabilities or
obligations hereunder.

          15.  Headings. The headings of the sections and paragraphs of this
               --------
Convertible Note are inserted for convenience only and do not constitute a part
of this Convertible Note.

          16.  Severability. If any provision of this Convertible Note is held
               ------------
invalid or unenforceable by any court of competent jurisdiction, the other
provisions of this Convertible Note will remain in full force and effect. Any
provision of this Convertible Note held invalid or unenforceable only in part or
degree will remain in full force and effect to the extent not held invalid or
unenforceable.

          17.  Cancellation. After all principal, premiums (if any) and accrued
               ------------
interest at any time owed on this Convertible Note have been paid in full, or
this Convertible Note has been converted in accordance with its terms, this
Convertible Note will be surrendered to the Company for cancellation and will
not be reissued.

          18.  Maximum Legal Rate. If at any time an interest rate applicable
               ------------------
hereunder exceeds the maximum rate permitted by law, such rate shall be reduced
to the maximum rate so permitted by law.

          19.  Place of Payment and Notices. Subject to Section 4(a) above,
               ----------------------------             ------------
payments of principal and interest and notices deliverable to the Payee
hereunder are to be delivered to the Payee at the following address: 299 Park
Avenue, New York, NY 10107, Attention: Mark Lama, or at such other address as
the Payee has specified by prior written notice to the Company. No notice shall
be deemed to have been delivered until the first Business Day following actual
receipt thereof at the foregoing address.

          20.  Waiver of Jury Trial. The Payee and the Company each hereby
               --------------------
waives any right it may have to a trial by jury in respect of any litigation
directly or indirectly arising out of, under or in connection with this
Convertible Note or the transactions contemplated hereunder.

          21.  Submission to Jurisdiction. (a) Any legal action or proceeding
               --------------------------
with respect to this Convertible Note may be brought in the courts of the State
of New York or of the United States of America for the Southern District of New
York, and, by execution and delivery of this Convertible Note, the Company
hereby accepts for itself and in respect of its property, generally and
unconditionally, the jurisdiction of the aforesaid courts.

          (b)  The Company hereby irrevocably waives, in connection with any
such action or proceeding, any objection, including, without limitation, any
objection to the laying of venue or based on the grounds of forum non
conveniens, which they may now or hereafter have to the bringing of any such
action or proceeding in such respective jurisdictions.

                                       -7-

<PAGE>

          (c)  Nothing herein shall affect the right of the Payee to serve
process in any other manner permitted by law or to commence legal proceedings or
otherwise proceed against the Company in any other jurisdiction.



          22.  GOVERNING LAW. ALL ISSUES AND QUESTIONS CONCERNING THE
               -------------
CONSTRUCTION, VALIDITY, ENFORCEMENT AND INTERPRETATION OF THIS CONVERTIBLE NOTE
SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF
NEW YORK, WITHOUT GIVING EFFECT TO ANY CHOICE OF LAW OR CONFLICT OF LAW RULES OR
PROVISIONS (WHETHER OF THE STATE OF NEW YORK OR ANY OTHER JURISDICTION) THAT
WOULD CAUSE THE APPLICATION OF THE LAWS OF ANY JURISDICTION OTHER THAN THE STATE
OF NEW YORK.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                                       -8-

<PAGE>

                  IN WITNESS WHEREOF, the Company has executed and delivered
this Convertible Promissory Note on the date first written above.

                                     IFX CORPORATION


                                     By: /s/ Michael Shalom
                                         ---------------------------------------
                                         Name:
                                         Title:

                                       -9-