<PAGE>

           NEITHER THIS DEBENTURE NOR THE SHARES OF COMMON STOCK ISSUABLE UPON
  CONVERSION OF THIS DEBENTURE HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF
  1933, AS AMENDED (THE "ACT") OR OTHERWISE QUALIFIED UNDER ANY STATE OR OTHER
  SECURITIES LAW, AND NEITHER MAY BE OFFERED, SOLD OR OTHERWISE TRANSFERRED,
  PLEDGED OR HYPOTHECATED UNLESS AND UNTIL REGISTERED UNDER THE ACT AND
  REGISTERED OR OTHERWISE QUALIFIED UNDER ANY APPLICABLE STATE OR OTHER
  SECURITIES LAW OR UNLESS THE COMPANY HAS RECEIVED AN OPINION OF COUNSEL OR
  OTHER EVIDENCE REASONABLY SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION
  IS NOT REQUIRED, EXCEPT AS PROVIDED HEREIN.

                            12% CONVERTIBLE DEBENTURE

 $3,000,000                                                    DECEMBER 28,1999

         FOR VALUE RECEIVED, U.S. RealTel, Inc., an Illinois corporation having
its principal place of business at 555 West Madison Street, Atrium Level South,
Chicago, Illinois 60661 (the "Company"), hereby promises to pay to the order of
Brandywine Operating Partnership, L.P., a Delaware limited partnership (the
"Holder"), at the place designated by the Holder, the principal amount of Three
Million Dollars ($3,000,000) (or such lesser amount as shall have been loaned to
the Company by the Holder pursuant to the Purchase Agreement defined below) and
to pay interest thereon on the terms set forth below. This 12% Convertible
Debenture (this "Debenture") is being delivered pursuant to the terms of that
certain Securities Purchase Agreement dated December 28, 1999 between the
Company and the Holder (the "Purchase Agreement"). All capitalized terms used
herein but not otherwise defined herein shall have the meanings ascribed to them
in the Purchase Agreement.

         1. PAYMENTS OF INTEREST AND PRINCIPAL. Interest shall accrue on the
outstanding principal balance hereof at a rate equal to twelve percent (12%) per
annum, payable quarterly in arrears, on the first day of each January, April,
July and October, commencing April 1, 2000 and continuing until the Maturity
Date (as defined below) at which time the principal balance and all accrued but
unpaid interest shall be paid in full. Interest shall be computed on the basis
of a 360-day year of twelve 30-day months.

         The Company agrees to pay interest (computed on the same basis as set
forth above) on overdue principal and on any overdue installment of interest at
the stated rate plus 3% per annum until paid. Notwithstanding the foregoing, the
Company shall not be required to pay interest in excess of the maximum rate
permitted by applicable law, and any payment in excess of such maximum rate
shall be credited against principal or refunded to the Company.

         All payments of principal and interest shall be made by the Company in
lawful money of the United States of America by wire transfer to an account
designated by the Holder; provided that the Holder may, from time to time, upon
twenty (20) days prior written notice to the Company, elect to receive one or
more installments of interest payable hereunder in duly authorized, validly
issued, fully paid and nonassessable shares of Common Stock, with each share
valued at $6.50 (subject to proportionate adjustment in the event of a split or
reverse split of the Common Stock or dividend on the Common Stock payable in
shares of Common Stock).


<PAGE>

         The term "Maturity Date" shall mean the earlier of (i) July 1, 2001 or
(ii) completion by the Company of an underwritten public offering of Common
Stock registered under the Act which yields net proceeds to the Company of at
least $10,000,000 (a "Qualified Offering"); provided that if the Company has not
completed a qualified Offering by July 1, 2001, it may, by providing the Holder
with written notice no later than 5:00 p.m. on July 1, 2001 (an "Extension
Notice"), extend the Maturity Date to the earlier of (i) October 1, 2001 or (ii)
completion by the Company of a Qualified Offering. If the Company provides the
Holder with an Extension Notice then the per share Exercise Price of the
Warrants issued to the Holder pursuant to the Purchase Agreement shall, as
provided in the Warrants, automatically be reduced by $1.50 from the per share
Exercise Price then in effect (with such $1.50 reduction being subject to
proportionate adjustment in the event of a split or reverse split of the Common
Stock or dividend on the Common Stock payable in shares of Common Stock).

         Upon satisfaction of the conditions contained in Section 5(b) hereof,
the Company shall have the right to prepay the principal amount hereof prior to
the Maturity Date.

         2. DEFAULT AND REMEDIES. The Company agrees that (a) if any interest is
not paid within five (5) days of the date on which the same is payable, (b)
should there be any breach by the Company in any material respect of any
representation, warranty or covenant set forth in the Purchase Agreement and
such breach remains uncured for a period of ten (10) business days following the
Company's receipt of written notice of such breach or (c) (i) if an event of
default occurs under that certain $1,500,000 Convertible Promissory Note dated
September 24, 1999 (the "September Note") issued by the Company and the holder
thereof accelerates the maturity thereof or (ii) if the principal balance of the
September Note is not repaid or converted into shares of Common Stock on the
terns set forth therein at the maturity date thereof (each an "Event of
Default"), the principal sum of this Debenture, with all accrued interest
thereon, shall, at the option of the Holder, become and be due and payable
immediately.

         3. ACCELERATION OPTION. Upon (a) the acquisition by a Person or group
of Persons acting in concert of beneficial ownership (as defined under Rule
13d-3 of the Exchange Act) of more than fifty percent (50%) of the outstanding
shares of Common Stock; (b) a sale, conveyance, exchange or transfer to another
Person not owned or controlled by the Company of all or substantially all of the
assets of the Company; (c) a merger or consolidation of the Company with one or
more other Persons; or (d) the merger or consolidation of one or more Persons
into or with the Company, if, in the case of (c) or (d), the shareholders of the
Company prior to such merger or consolidation do not retain at least a majority
of the voting power of the Company or surviving Person, as the case may be, the
principal sum due hereunder, with all accrued interest thereon, shall, at the
option of the Holder, become and be due and payable immediately. The Holder may
exercise such acceleration right at any time prior to the later of (i)
consummation of any such event and (ii) the Holder's receipt of written notice
from the Company advising the Holder of the pending or completed consummation of
such event.

         4. TRANSFER. This Debenture and the rights granted to the Holder are
transferable, in whole or in part, upon surrender of this Debenture, together
with an executed instrument of assignment, at the address of the Company
specified in Section 8 hereof. Until due presentment for registration of
transfer on the books of the Company, the Company may treat the


                                       2
<PAGE>

registered holder hereof as the owner hereof for all purposes, and the Company
shall not be affected by any notice to the contrary. The Company may require, as
a condition of allowing such transfer, that the Holder or transferee of this
Debenture furnish to the Company a written opinion of counsel, in form,
substance and scope customary to opinions typically delivered in transactions of
this nature, to the effect that such transfer may be made without registration
under the Act and under applicable state securities laws; provided that no such
opinion shall be required if the transferee is a direct or indirect subsidiary
of Brandywine Operating Partnership, LP: or Brandywine Realty Services
Corporation.

         5. CONVERSION.

                  (a) OPTIONAL CONVERSION. The Holder shall have the right, at
the Holder's option, at any time and from time to time prior to repayment of all
amounts due under this Debenture, to convert all or any portion of the
outstanding principal amount of this Debenture into that number of shares (the
"Conversion Shares") of Common Stock equal to the quotient obtained by dividing
the principal amount of this Debenture being converted by a conversion price
equal initially to $7.50 per share (the "Conversion Price"), which Conversion
Price shall be subject to adjustment as set forth in Section 6.

                  (b) PREPAYMENT. At such time and from time to time as (i) a
public market for the Common Stock exists and (ii) the Market Price (as defined
below) of the Common Stock has equaled or exceeded 120% of the then effective
Conversion Price for twenty (20) out of thirty (30) consecutive Trading Days (as
defined below), the Company shall have the right to prepay this Note without
premium or penalty on not less than thirty (30) days' prior written notice to
the Holder (the last day of such thirty (30) day period being referred to as the
"Prepayment Date"). Upon receipt of such prepayment notice, the Holder shall
have the option, at its sole election, by providing the Company with written
notice thereof prior to the Prepayment Date, to convert all or any portion of
the outstanding principal of this Debenture into Conversion Shares at the
Conversion Price. The term "Market Price" shall mean, for any given Trading Day,
the average of the closing bid and asked prices of the Common Stock quoted in
the Over-The-Counter Market Summary or the last reported sale price of the
Common Stock or the closing price quoted on the NASDAQ National Market System or
on any exchange on which the Common Stock is listed, whichever is applicable, as
published in the Eastern Edition of The WALL STREET JOURNAL or, if not reported
in THE WALL STREET JOURNAL, as reported by Bloomberg. The term "Trading Day"
means a day on which banks in Chicago, Illinois are open and on which at least
1,000 shares of Common Stock are traded between unaffiliated persons or
entities.

                  (c) CONVERSION PROCEDURE. In order to exercise the conversion
right set forth in Sections 5(a) and 5(b), the Holder shall surrender this
Debenture, duly endorsed (or, in the event that such Debenture has been lost,
stolen or destroyed, the Holder shall execute an agreement reasonably
satisfactory to the Company to indemnify the Company from any loss incurred by
it resulting from the fact that such Debenture has been lost, stolen or
destroyed) to the Company at its address set forth in Section 8 hereof, together
with written notice that the Holder elects to convert this Debenture or the
portion thereof specified in said notice. As promptly as practicable after the
surrender of this Debenture as aforesaid, in full or in part, and in any event
within ten (10) days thereafter, the Company, at its expense, shall issue and
deliver to


                                       3
<PAGE>

the Holder a certificate or certificates for the number of full shares of Common
Stock issuable upon the conversion of this Debenture or portion thereof
registered in the name of the Holder and a check or cash in respect of any
fractional interest in respect of a share of Common Stock arising upon such
conversion, as provided below. In case this Debenture shall be surrendered for
partial conversion, the Company shall execute and deliver to the Holder, without
charge, a new Debenture in an aggregate principal amount equal to the
unconverted portion of the surrendered Debenture, provided that, except for the
amount of shares into which the new Debenture may be converted, the new
Debenture shall have all of the same terms and conditions as this Debenture.

                  (d) EFFECTIVE DATE OF CONVERSION. Each conversion pursuant to
Section 5(a) hereof shall be deemed to have been effected immediately prior to
the close of business on the day on which this Debenture shall have been
surrendered, as aforesaid, and the Holder shall be deemed to have become on said
date the holder of record of the shares of Common Stock issuable upon such
conversion.

                  (e) FULLY PAID SHARES. All shares of Common Stock issued upon
conversion of all or any portion of this Debenture shall be duly authorized,
validly issued, fully paid, non-assessable, and free and clear of all claims,
liens or encumbrances and, if any other outstanding shares of Common Stock are
then listed on a national or regional securities exchange, will be so listed.

                  (f) NO FRACTIONAL SHARES. No fractional shares of Common Stock
or scrip representing fractional shares shall be issued upon conversion of this
Debenture. If any fractional share of Common Stock would be issuable upon the
conversion of this Debenture, then the Company shall make an adjustment therefor
in cash at the Conversion Price.

                  (g) ACCRUED INTEREST. Upon any conversion of this Debenture,
or any portion hereof, appropriate cash adjustment shall be made, for or on
account of any interest accrued hereon on such portion. Such accrued interest
shall be paid in cash or, at the option of the Holder, in shares of Common Stock
valued as provided in Section 1.

         6. ADJUSTMENTS. The Conversion Price and the number of shares of Common
Stock purchasable upon conversion of this Debenture are subject to adjustment
from time to time as follows:

                  (a) INCURRENCE OF CERTAIN INDEBTEDNESS. In the event that the
Company, directly or through a Subsidiary, hereafter incurs, assumes or
guaranties any Indebtedness (as defined below) that does not constitute Eligible
Indebtedness (as defined below), the Conversion Price shall, as of the date of
the incurrence or guaranty, decrease automatically to $1.00 less than the
Conversion Price in effect immediately prior to such incurrence or guaranty
(with such $1.00 decrease being subject to proportionate adjustment in the event
of a split or reverse split or dividend on the Common Stock payable in shares of
Common Stock prior to any such decrease). As used herein, (i) the term
"Indebtedness" means indebtedness for borrowed money and (ii) the term "Eligible
Indebtedness" means Indebtedness which is (a) Indebtedness incurred after the
date hereof to refinance Indebtedness outstanding on the date hereof (provided
that the principal amount of such new Indebtedness does not exceed


                                       4
<PAGE>

the principal amount of the Indebtedness being refinanced), (b) Indebtedness
which, by its terms and except for scheduled interest, is not entitled to be
repaid (other than by the issuance of the Company's non-redeemable equity
securities or equity securities that are not subject to redemption prior to
payment in full or conversion in full of this Debenture including accrued
interest) prior to payment in full or conversion in full of this Debenture,
including accrued interest, (c) Indebtedness the proceeds of which are used
exclusively to acquire property that secures repayment of such Indebtedness and
which Indebtedness is otherwise non-recourse to the assets and credit of the
Company and it Subsidiaries or (d) Indebtedness incurred, assumed or guaranteed
after the date hereof which, together with all other Indebtedness incurred,
assumed or guaranteed after the date hereof, does not exceed, in the aggregate,
$1,500,000 (exclusive of Indebtedness that falls within any of clauses (a), (b)
or (c)).

                  (b) STOCK DIVIDEND, SPLIT OR SUBDIVISION OF SHARES. If the
number of shares of Common Stock outstanding at anytime after the date hereof is
increased or deemed increased by a stock dividend payable in shares of Common
Stock or other securities convertible into or exchangeable for shares of Common
Stock ("Equivalents") or by a subdivision or split-up of shares of Common Stock
or Equivalents (other than a change in par value, from par value to no par value
or from no par value to par value), then, following the effective date fixed for
the determination of holders of Common Stock or Equivalents entitled to receive
such stock dividend, subdivision or split-up, the Conversion Price shall be
proportionately decreased (but in no event shall the Conversion Price be
decreased below the par value of the Common Stock issuable upon conversion of
this Debenture).

                  (c) COMBINATION OF SHARES. If, at any time after the date
hereof, the number of shares of Common Stock outstanding is decreased by a
combination of the outstanding shares of Common Stock (other than a change in
par value, from par value to no par value or from no par value to par value),
then, following the effective date for such combination, the Conversion Price
shall be proportionately increased.

                  (d) REORGANIZATIONS. CONSOLIDATIONS, ETC. In the event, at any
time after the date hereof, of any capital reorganization, or any
reclassification of the capital stock of the Company (other than a change in par
value or from par value to no par value or from no par value to par value or as
a result of a stock dividend or subdivision, split-up or combination of shares),
or the consolidation or merger of the Company with or into another Person (other
than a consolidation or merger in which the Company is the continuing
corporation and which does not result in any change in the powers, designations,
preferences and rights, or the qualifications, limitations or restrictions, if
any, of the capital stock of the Company as amended from time to time) or of the
sale or other disposition of all or substantially all the properties and assets
of the Company in its entirety to any other Person (any such transaction, an
"Extraordinary Transaction"), then this Debenture, if not repaid in full at the
time of such Extraordinary Transaction pursuant to Section 3 hereof, shall be
convertible into the kind and number of shares of stock or other securities or
property of the Company, or of the Person resulting from or surviving such
Extraordinary Transaction, that a holder of the number of shares of Common Stock
deliverable (immediately prior to the effectiveness of the Extraordinary
Transaction) upon conversion of this Debenture would have been entitled to
receive upon such Extraordinary Transaction, provided that following any such
Extraordinary Transaction provision is made to


                                       5
<PAGE>

continue the application of the adjustments contained in this Section 5 as
nearly as reasonably may be.

                  (e) CERTIFICATE AS TO ADJUSTMENTS. Upon the occurrence of each
adjustment or readjustment pursuant to this Section 6, the Company at its own
expense shall promptly compute such adjustment or readjustment in accordance
with the terms hereof and furnish to each the holder hereof a certificate
setting forth such adjustment or readjustment and showing in detail the facts
upon which such adjustment or readjustment is based. The Company shall, upon the
written request, at any time, of any such holder, furnish or cause to be
furnished to such holder a like certificate setting forth: (i) such adjustments
and readjustments; (ii) the Conversion Price at the time in effect; and (iii)
the number of shares that at the time would be received upon the conversion of
this Debenture.

         7. NOTICES. In case at any time:

                  (a) the Company shall declare any cash dividend upon its
Common Stock;

                  (b) the Company shall declare any dividend upon its Common
Stock payable in stock or make any special dividend or other distribution (other
than cash dividends) to the holders of Common Stock;

                  (c) the Company shall offer for subscription pro rata to the
holders of its Common Stock any additional shares of stock of any class or other
rights;

                  (d) there shall be any capital reorganization or
reclassification of the capital stock of the Company, or consolidation or merger
of the Company with, or sale of all or substantially all of its assets to,
another Person; or

                  (e) there shall be a voluntary or involuntary dissolution,
liquidation or winding up of the Company;

then, in any one or more of said cases, the Company shall give the Holder (i) at
least 10 business days prior written notice of the date on which the books of
the Company shall close or a record shall be taken for such dividend,
distribution or subscription rights or for determining rights to vote in respect
of any such reorganization, reclassification, consolidation, merger, sale,
dissolution, liquidation or winding up and (ii) in the case of any such
reorganization, reclassification, consolidation, merger, sale, dissolution,
liquidation or winding up, at least 10 business days prior written notice of the
date when the same shall take place. Such notice in accordance with the
foregoing clause (i) shall also specify, in the case of any such dividend,
distribution or subscription rights, the date on which the holders of Common
Stock shall be entitled thereto, and such notice in accordance with the
foregoing clause (ii) shall also specify the date on which the holders of Common
Stock shall be entitled to exchange their Common Stock for securities or other
property deliverable upon such reorganization, reclassification, consolidation,
merger, sale, dissolution, liquidation or winding up, as the case may be.


                                       6
<PAGE>

         8. COMMUNICATIONS. All notices or requests to be given pursuant to this
Debenture must be in writing and may be given as follows:

         The Company:

                  US RealTel, Inc.
                  555 West Madison Street
                  Atrium Level South
                  Chicago, IL 60661
                  Attention: Ilene Dobrow Davidson, Esq.
                  Telephone: 312-775-8900
                  Telecopier. 312-756-9016

         with a copy to:

                  Sachnoff & Weaver
                  30 South Wacker Drive
                  29th Floor
                  Chicago, IL 60606-7478
                  Attention: Seth M. Hemming, Esq.
                  Telephone: 312-207-1000
                  Telecopier: 312-207-6400

         If to the Holder:

                  Brandywine Realty Trust
                  14 Campus Boulevard
                  Suite 100
                  Newtown Square, PA 19073
                  Attention: Gerard H. Sweeney
                  Telephone: 610-325-5600
                  Telecopier: 610-325-5622

         with a copy to:

                  Brandywine Realty Trust
                  14 Campus Boulevard
                  Suite 100
                  Newtown Square, PA 19073
                  Attention: Brad A. Molotsky, Esq.
                  Telephone: 610-325-5600
                  Telecopier. 610-325-5622


                                       7
<PAGE>

         with an additional copy to:

                  Pepper Hamilton LLP
                  3000 Two Logan Square
                  18th and Arch Streets
                  Philadelphia, PA 19103
                  Attention: Michael H. Friedman, Esq.
                  Telephone: 215-981-4563
                  Telecopier: 215-981-4750

         Any notice shall be deemed to have been given if personally delivered
or sent by United States mail or by commercial courier or delivery service or by
telecopier or telegram and shall be deemed received, unless earlier received,
(i) if sent by certified or registered mail, return receipt requested, three
business days after deposit in the mail, postage prepaid (ii) if sent by United
States Express Mail or by commercial courier or delivery services, on business
day after delivery to a United States Post Office or delivery service, postage
prepaid, (iii) if sent by telecopier or telegram, when receipt is acknowledged
by answerback, and (iv) if delivered by hand, on the date of the receipt.

         9. COMPANY'S WAIVERS. The Company, to the extent permitted by law,
waives and agrees not to assert or take advantage of any of the following: (a)
acceptance or notice of acceptance of this Debenture by the Company; (b)
presentment and/or demand for payment of this Debenture or any indebtedness or
obligations hereby promised; and (c) protest and notice of dishonor with respect
to this Debenture or any indebtedness or performance of obligations arising
hereunder.

         10. GOVERNING LAW. This Debenture shall be governed by and construed in
accordance with the laws of the State of Illinois, without regard to conflicts
of law principles.

         11. HEADINGS. The Section headings of this Debenture are inserted for
convenience only and do not constitute a pact of this Debenture.

            IN WITNESS WHEREOF, the Company, intending to be legally bound
hereby, has caused this Debenture to be duly executed effective as of the day
and year first above written.

                                            U.S. REALTEL, INC.

                                            By:  /s/ Perry H. Ruda
                                               -----------------------------
                                                 Name:  Perry H. Ruda
                                                 Title: Chairman


                                       8