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                                                                   EXHIBIT 10.21



                         PENTASTAR COMMUNICATIONS, INC.

                                 PROMISSORY NOTE

U.S. $142,000                                                  December 29, 2000


         FOR VALUE RECEIVED, PENTASTAR COMMUNICATIONS, INC., a Delaware
corporation (the "Company"), hereby promises to pay to the order of Robert S.
Lazzeri, the principal sum of One Hundred Forty Two Thousand and No/100ths
United States Dollars (U.S. $142,000) or so much thereof as may have been
advanced to the Company by the payee and outstanding from time to time, with
interest thereon at the rate of twelve percent (12%) per annum from the date
advanced to the Company until paid in full.

         This Note is subject to the following terms and conditions:

         I. COMPUTATION AND PAYMENT OF INTEREST. Interest will be computed on
the basis of a 360-day year and the actual number of days elapsed. Accrued and
unpaid interest will be added to the outstanding principal balance of this Note
on March 31, 2001 and on the last day of each calendar quarter thereafter to and
including December 31, 2002. Accrued interest on the outstanding principal
balance of this Note shall be payable quarterly commencing March 31, 2001 and
continuing on the last day of each calendar quarter thereafter until this Note
is paid in full; provided however, in the event that interest may not be paid on
any such payment date as a result of the provisions of credit facilities
extended by Wells Fargo Bank, N.A. to the Company, the accrued and unpaid
interest at such interest rates shall be added to the outstanding principal
balance of this Note on each of the foregoing payment dates.

         II. METHOD OF PAYMENT. The Company will pay principal and interest in
United States dollars. Payments of principal and interest shall be made by wire
transfer to an account designated from time to time by the payee or other holder
of this Note (the "Holder"). If the Holder fails to designate such an account,
payments shall be made by check at the address of the Holder as reflected on the
records of the Company. The Holder must surrender this Note to the Company at
the time of payment in full of all accrued interest and outstanding principal.

         III. PAYMENT; PREPAYMENT. All accrued interest and outstanding
principal evidenced by this Note shall be due and payable in full on the
earliest of December 31, 2002, acceleration of the amount evidenced by this Note
as provided herein or the occurrence of a Change of Control (as defined below).
The Company may voluntarily prepay this Note in whole or in part at any time and
from time to time. Change of Control means any transaction or series of
transactions pursuant to which a third party or a group of third parties acting
in concert (excluding the shareholders of the Company as of the date of this
Note) acquires (i) at least fifty percent (50%) of the fully diluted common
stock of the Company, whether through purchase, merger,


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consolidation or otherwise or (ii) at least fifty percent (50%) of the Company's
operating assets, based on fair market value as a going concern. The Company
shall give the Holder notice of the occurrence of a Change of Control
immediately upon the occurrence thereof. Notwithstanding anything to the
contrary in this Note or in the Promissory Note dated as of the date hereof in
the principal amount of $163,000 to the order of Richard M. Tyler or the
Promissory Note dated as of the date hereof in the principal amount of $345,000
to the order of Craig J. Zoellner (collectively, such notes together with this
Note are referred to as the "Pentastar Notes"), any payment or prepayment by the
Company on any Pentastar Note shall be made only if payments are made
concurrently on all of the other Pentastar Notes pro-rata in accordance with the
outstanding principal amount of such Pentastar Notes.

         IV. APPLICATION OF PAYMENTS. All payments received on this Note shall
be applied first to the payment of fees and expenses due hereunder, then to the
payment of accrued interest and then to the reduction of principal.

         V. WAIVERS. The Company and each endorser and guarantor of this Note,
and any other person who is now or may hereafter become primarily or secondarily
liable for or provide collateral security for the payment of this Note or any
portion thereof (a) waives presentment, notice of dishonor and protest, (b)
agrees to one or more extensions of time of payment of all or any part of this
Note, for any length of time and (c) agrees that the Holder may release, agree
not to sue, suspend its rights to enforce this Note against, or otherwise
discharge or deal with any person against whom such maker, endorser, guarantor
or other person has a right of recourse, and may release, fail or agree not to
enforce or perfect its rights in or against, or otherwise deal with collateral
for the payment of this Note or any portion thereof.

         VI. ATTORNEYS' FEES. If this Note or interest thereon is not paid when
due, or suit is brought to enforce any right hereunder, the Company and each
endorser and guarantor of this Note and any other person who is now or may
hereafter become primarily or secondarily liable for the payment of this Note or
any portion thereof agrees to pay all reasonable costs of collection and
enforcement, including reasonable attorneys' fees. In the event of any
bankruptcy or insolvency proceedings involving the Company, costs of collection
shall include all costs and attorneys' fees incurred in connection with such
proceedings, including the fees of counsel for attendance at meetings of
creditors.

         VII. INTEREST AFTER DUE DATE. If the principal of or any accrued
interest on this Note is not paid when due, as a result of acceleration or
otherwise, the overdue amount shall bear interest until paid at the rate of
fifteen percent (15%) per annum, compounded monthly until paid.

         VIII. INTEREST SAVINGS CLAUSE. Notwithstanding any other provision
hereof, the interest payable hereunder shall be limited to the maximum amount
permitted under applicable law. If any amount is paid hereunder which would be
usurious under applicable law, it shall be


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deemed a prepayment of principal or shall be promptly refunded to the Company as
necessary to avoid violation of any applicable usury statute.

         IX. DEFAULT AND ACCELERATION.

             (a) EVENT OF DEFAULT. An "Event of Default" occurs if:

                 (i) any payment of interest on or principal of this Note is not
made when it becomes due hereunder, whether at maturity or otherwise, including
without limitation as a result of the requirements of Section III above;

                 (ii) pursuant to or within the meaning of any bankruptcy or
other law for the relief of debtors in any jurisdiction:

                      (A) proceedings are commenced by or against the Company;

                      (B) a receiver, trustee, assignee, liquidator, conservator
or similar official is appointed for the Company;

                      (C) the Company makes a general assignment for the benefit
of its creditors; or

                      (D) the liquidation of the Company is ordered by a court;
provided, however, that if an event described in (A) or (B) above occurs other
than at the instance or with the consent or acquiescence of the Company, it
shall not constitute an Event of Default unless the Company fails, within thirty
(30) days after its occurrence, to have the proceedings dismissed (or stayed
continuously until dismissal) or the appointment revoked, vacated or otherwise
terminated;

                 (iii) the Company ceases the active conduct of its business or
sells or otherwise disposes of all or substantially all of its assets;


                 (iv) the Company is dissolved or liquidated, the charter of the
Company is surrendered or revoked or the Company takes any action in
contemplation of any thereof; or

                 (v) an event of default occurs and is continuing under credit
facilities extended by Wells Fargo Bank, N.A. to the Company, the WCMA Reducing
Revolving Loan And Security Agreement dated June 28, 2000 by Merrill Lynch
Business Financial Services, Inc. to the Company, or replacements or
refinancings of the foregoing.

             (b) ACCELERATION. Upon the occurrence of an Event of Default
specified in Section IX(a)(iii) above, all of the outstanding principal and
accrued and unpaid interest


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immediately shall be due and payable in full. If any other Event of Default
occurs and is continuing, the Holder may accelerate the maturity of this Note.
Upon such acceleration, the principal of and all accrued interest on this Note
shall be due and payable immediately. The Holder may at any time rescind any
such acceleration and its consequences.

             (c) OTHER REMEDIES. If an Event of Default occurs and is
continuing, the Holder may pursue any available remedy to collect the payment of
principal or interest on this Note or to enforce the performance of any
provision of this Note. A delay or omission by the Holder in exercising any
right or remedy accruing upon an Event of Default shall not impair the right or
remedy or constitute a waiver of or acquiescence in the Event of Default. All
remedies are cumulative.

         X. NOTICES. All notices that are required or may be given hereunder
shall be in writing and may be given by personal delivery or by facsimile
transmission or mail. The person sending any notice shall prepay all
transmission charges. Any notice personally delivered or given by mail shall be
deemed effective upon receipt. Transmission by a recognized courier service
shall be deemed personal delivery and any notice so delivered shall be deemed
received at the time of delivery confirmed by the courier service. Any notice
sent by facsimile transmission shall be deemed received upon confirmation of
transmission by the sender's facsimile machine. The following addresses and
telecopy numbers may be used for delivery of notices until another address or
telecopy number is specified by the Company or the Holder in a written notice to
the other:

<TABLE>
<S>                                         <C>

         If to the Company:                 Pentastar Communications, Inc.
                                            1660 Wynkoop Street, Suite 1010
                                            Denver, Colorado 80202
                                            Attention:  Chief Executive Officer
                                            Telecopy: 303-825-4402

         If to the Holder:                  Robert S. Lazzeri
                                            c/o PentaStar Communications, Inc.
                                            1660 Wynkoop, Suite 1010
                                            Denver, CO 80202 Telecopy:
                                            303-825-4402

</TABLE>

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         XI. SUBORDINATION. Anything herein to the contrary notwithstanding, the
indebtedness evidenced by this Note (or any renewal or extension thereof),
including principal, premium, if any, and interest, is and shall be subordinate
in right of payment, to the prior payment in full of all indebtedness of the
Company now outstanding or hereafter incurred to Wells Fargo, N.A., its
successors or assigns (the "Wells Fargo Debt"). The Holder of this Note
acknowledges that the Wells Fargo Debt incurred after the date hereof may
include, without limitation, bank or other borrowings in connection with working
capital lines of credit and future business acquisitions that would materially
increase the amount of Wells Fargo Debt.

         XII. GOVERNING LAW; JURISDICTION AND VENUE. This Note shall be governed
by the laws of the State of Colorado. The Company hereby submits to the personal
jurisdiction of the state and federal courts of appropriate subject matter
jurisdiction located in the State of Colorado, for the purposes of any suit to
enforce any rights of the Holder under this Note. The Company waives any right
to object to suit in any such court on the grounds of an inconvenient forum or
otherwise. The Holder may, at his option, bring any such suit in any other court
with jurisdiction over the Company.

                                            PENTASTAR COMMUNICATION, INC.



                                            By: /s/ Robert S. Lazzeri
                                                --------------------------------
                                            Name: Robert S. Lazzeri
                                            Its:  Chief Executive Officer

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