<PAGE>   1
                                                                   EXHIBIT 10.33


                        FOUNDER STOCK PURCHASE AGREEMENT

     This Founder Stock Purchase Agreement is dated as of the 23rd day of
December, 1997 (the "Effective Date") by and between MachOne Communications,
Inc., a California corporation (the "Company"), and Thomas Obenhuber
("Founder").


                                  WITNESSETH:

     WHEREAS, Founder is a founder and a key employee of the Company.

     WHEREAS, the Company desires to issue and the Founder desires to acquire
stock of the Company as herein described, on the terms and conditions
hereinafter set forth.

     NOW, THEREFORE, IT IS AGREED between the parties as follows:

     1.   Number of Shares and Price Per Share. The Founder hereby agrees to
purchase from the Company and the Company agrees to sell to the Founder 240,000
shares of the Company's Common Stock (the "Stock") with a fair market value of
$240.00 (or $0.001 per share). The consideration for the Stock (the "Purchase
Price") will be paid by Purchaser in cash, by check or promissory note
concurrent with the execution of this Agreement against the Company's delivery
of a stock certificate evidencing the Stock.

     2.   Unvested Share Repurchase Option. The Company shall have the option
(the "Unvested Shares Repurchase Option") to reacquire any shares purchased
pursuant to this Agreement which have not vested in the Founder pursuant to
subsection 2(a) (the "Unvested Shares") under the terms set forth in this
Section 2.

          (a)  Vesting Shares. The "Initial Vesting Date" shall be February 3,
1998. The shares of Stock purchased by the Founder will vest (the "Vested
Shares") on and after the Initial Vesting Date in accordance with the following
formula:

<TABLE>
<CAPTION>
          Date                                    Number of Shares Vested
          ----                                    -----------------------
          <S>                                     <C>
          On the Initial Vesting Date             30,000 shares of stock will
                                                  vest

          For each of the following 42 full       An additional 2.0833% of the
          month of the Company's continuous       Stock will vest (5,000 shares)
          employment of Founder following the     for each full month of service.
          Initial Vesting Date
</TABLE>

     Provided that the aggregate number of shares of Stock constituting Vested
Shares may not exceed 240,000 shares (as adjusted for stock splits and the
like). In the event a fraction of a share is vested, the number of vested
shares shall be rounded to the nearest whole number.



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     (b)  Exercise of Unvested Share Repurchase Option. If the Founder's
employment with the Company is terminated for any reason, with or without
cause, voluntarily or involuntarily, including termination due to death or
disability (as defined below), or if the Founder or the Founder's legal
representative attempts to dispose of any Unvested Shares other than as allowed
in this Agreement, the Company may exercise the Unvested Share Repurchase
Option by written notice to the Escrow Agent (as defined in Section 8) and to
the Founder or the Founder's legal representative within 60 days after such
termination or after the Company has received notice of the attempted
disposition.

     (c)  Payment for Shares and Return of Shares. Payment by the Company to
the Escrow Agent on behalf of the Founder or the Founder's legal representative
shall be made in cash within 60 days after the date of the mailing of the
written notice of exercise of the Unvested Share Repurchase Option. For
purposes of the foregoing, cancellation of any promissory note of the Founder
to the Company shall be treated as payment to the Founder in cash to the extent
of the unpaid principal and any accrued interest canceled. The purchase price
per share being purchased by the Company pursuant to the Unvested Share
Repurchase Option shall be $0.001 per share, adjusted appropriately to reflect
any stock split, stock dividend, recapitalization, etc. Within 30 days after
payment by the Company, the Escrow Agent shall give the shares which the
Company has purchased to the Company and shall give the payment received from
the Company to the Founder.

     (d)  Early Termination of Unvested Share Repurchase Option. The other
provisions of Section 2 notwithstanding, upon any Transfer of Control (as
defined below), the Unvested Share Repurchase Option shall terminate as of a
date prior to the Transfer of Control, as the Board so determines, or if no
such determination is made, two days prior to the closing of the transaction
involving the Transfer of Control. Any such termination that was permissible
solely by reason of this subsection 2(d) shall be conditioned upon the
consummation of the Transfer of Control. For purposes of this subsection 2(d),
a Transfer of Control shall be deemed to have occurred upon any of the
following events: (i) the direct or indirect sale or exchange by the
shareholders of the Company of all or substantially all of the stock of the
Company where the shareholders of the Company before such sale or exchange do
not retain, directly or indirectly, at least a majority of the beneficial
interest in the voting stock of the Company; (ii) a merger in which the
shareholders of the Company before the merger do not retain, directly or
indirectly, at least a majority of the beneficial interest in the voting stock
of the Company; or (iii) the sale, exchange, or transfer of all or
substantially all of the Company's assets (other than a sale, exchange, or
transfer to one or more corporations where the shareholders of the Company
before such sale, exchange, or transfer retain, directly or indirectly, at
least a majority of the beneficial interest in the voting stock of the
corporation(s) to which the assets were transferred).

     (e)  Transfers Not Subject to the Unvested Share Repurchase Option. The
Unvested Share Repurchase Option shall not apply to a transfer of shares of the
Stock to the Founder's ancestors, descendants or spouse or to a trustee for
their benefit or the benefit of the Founder, provided that such transferee
shall agree in writing (in a form satisfactory to the Company) to take the
shares of the Stock subject to all the terms and conditions of this Section 2.

     (f)  Assignment of Unvested Share Repurchase Option. The Company may
assign the Unvested Share Repurchase Option to one or more persons, who shall
have the right to


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<PAGE>   3
exercise the Unvested Share Repurchase Option in his or her own name for his or
her own account.

     3.  Right of First Refusal. Before any shares of the Stock registered in
the name of Founder may be sold or transferred (including transfer by operation
of law), such shares shall first be offered to the Company, which will have the
right to purchase all or any part of such shares proposed to be transferred
("Right of First Refusal"), in the following manner:

          (a)  Transfer Notice. The Founder or his or her legal representative
shall first give written notice (the "Transfer Notice") of any proposed
transfer to the Company. The Transfer Notice shall name the proposed
transferee, state the number of shares of Stock to be transferred, the price
per share and all other terms of the offer. The Transfer Notice shall be signed
by the Founder or his or her representative and the prospective transferee and
must constitute a binding agreement for the transfer of the Stock subject only
to the Right of First Refusal.

          (b)  Bona Fide Determination. Within 30 days of delivery of the
Transfer Notice, the Company's Board of Directors shall determine the bona fide
nature of the proposed transfer and give the Founder written notice of its
determination. If the proposed transfer is deemed to be bona fide, the
remaining subsections of this section shall apply to the sale. If the proposed
transfer is deemed not to be bona fide, the Founder will be responsible for
providing additional information to the Board to show the bona fide nature of
the proposed transfer and no Stock will be transferred on the books of the
Company until the Board has approved the proposed transfer as bona fide.

          (c)  Failure to Exercise; Exercise. If the Company elects not to or
fails to exercise in full the Right of First Refusal within 30 days from the
later of the date the Transfer Notice is delivered to the Company or 30 days
after the date the transfer is determined to be bona fide (if the Founder is
required to provide additional information as provided in Section 3(b)), the
Founder may, by the later of 60 days after the delivery of the Transfer Notice
to the Company or 30 days after the date the transfer is determined to be bona
fide (if the Founder is required to provide additional information as provided
in Section 3(b)), conclude a transfer of the shares of Stock subject to the
Transfer Notice which have not been purchased by the Company pursuant to
exercise of the Right of First Refusal on the terms and conditions described in
the Transfer Notice. Any proposed transfer on terms and conditions different
from those described in the Transfer Notice, as well as any subsequent proposed
transfer by the Founder, shall again be subject to the Right of First Refusal
and shall require compliance by the Founder with the procedure described in
this Section 3. If the Company exercises the Right of First Refusal, the
parties shall consummate the sale of shares of Stock on the terms set forth in
the Transfer Notice by the later of 60 days after the delivery of the Transfer
Notice to the Company or 30 days after the date the transfer is determined to
be bona fide (if the Purchaser is required to provide additional information as
provided in Section 3(b)); provided, however, in the event the Transfer Notice
provides for the payment for the shares of Stock other than in cash, the
Company shall have the option of paying for the shares of Stock by the
discounted cash equivalent of the consideration described in the Transfer
Notice as reasonably determined by the Company.

          (d)  Condition to Transfer. All transferees of shares of Stock or any
interest therein other than the Company shall be required as a condition of
such transfer to agree in



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writing (in a form satisfactory to the Company) that they will receive and hold
such shares of Stock or interests subject to the provisions of this Agreement,
including the Right of First Refusal.

            (e)   Assignment of Right of First Refusal. The Company may assign
the Right of First Refusal to one or more persons, who shall have the right to
exercise the Right of First Refusal in his or her own name for his or her own
account.

            (f)   Termination. The Right of First Refusal will terminate upon
the closing of a firm commitment underwritten public offering to the public of
the Company's Common Stock pursuant to a registration statement under the
Securities Act of 1933, as amended (the "IPO").

            (g)   Transfer Not Subject to Right of First Refusal. The Right of
First Refusal shall not apply to a transfer of shares of the Stock to the
Founder's ancestors, descendants or spouse or to a trustee for their benefit or
the benefit of the Founder, provided that such transferee shall agree in
writing (in a form satisfactory to the Company) to take the shares of Stock
subject to all the terms and conditions of this Section 3.

      4.    Piggyback Registration Rights.

            (a)   If the Company shall determine to register any of its
securities either for its own account or the account of a shareholder(s)
exercising demand registration rights, other than a registration relating
solely to employee benefit plans, or a registration relating solely to a
transaction pursuant to Rule 145 promulgated under the Securities Act of 1933,
or a registration on any registration form which does not permit secondary
sales or does not include substantially the same information as would be
required to be included in a registration statement covering the sale of the
Stock, the Company will promptly give to the Founder written notice thereof and
include in such registration (and any related qualification under blue sky
laws), and in any underwriting involved therein, the number of Vested Shares
specified in a written request made by the Founder within fifteen (15) days
after receipt of such written notice from the Company, except as set forth in
Section 4(b) below.

            (b)   If the registration of which the Company gives notice is for a
registered public offering involving an underwriting, the right of any Founder
to registration shall be conditioned upon the Founder's participation in such
underwriting and the inclusion of such Founder's Stock in the underwriting
pursuant to an underwriting agreement in customary form with the underwriter or
underwriters pursuant to an underwriting agreement in customary form with the
underwriter or underwriters selected by the Company. Notwithstanding any other
provision of this Section, if the underwriter reasonably determines that
marketing factors require a limitation on the number of shares to be
underwritten the underwriter may exclude some or all of the Stock with the
number of shares that may be included in the registration and underwriting being
allocated among the Founder and all other shareholders entitled to have
securities included in such registration in proportion, as nearly as
practicable, to the respective amounts of securities which they had requested to
be included in such registration (provided, however, that if the registration is
for the account of shareholders exercising demand registration rights, the
number of shares that may be included by the Founder shall be cut back entirely
before any limitation on the number of shares that may be included by such
shareholders).



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            (c)   All expenses of the registration shall be borne by the
Company, except underwriting discounts and selling commissions applicable to
the sale of any of Founder's Stock and any other securities of the Company
being sold in the same registration by other shareholders, which shall be borne
by the Founder and such other shareholders pro rata on the basis of the number
of their shares registered.

      5.    Stock Dividends, etc. If, from time to time, there is any stock
dividend, stock split or other change in the character or amount of any of the
outstanding stock of the Company, then in such event any and all new
substituted or additional securities to which Founder is entitled by reason of
Founder's ownership of Unvested Shares or Stock shall be immediately subject to
the Unvested Share Repurchase Option or the Right of First Refusal,
respectively, with the same force and effect as the Unvested Shares or Stock.

      6.    Consent of Spouse. If the Founder is married on the date of this
Agreement, the Founder's spouse shall execute a Consent of Spouse in the form
of Exhibit A hereto, effective on the date hereof. Such consent shall not be
deemed to confer or convey to the spouse any rights in the Stock that do not
otherwise exist by operation of law or the agreement of the parties. If the
Founder should marry or remarry subsequent to the date of this Agreement, the
Founder shall within thirty (30) days thereafter obtain his or her new spouse's
acknowledgement of and consent to the existence and binding effect of all
restrictions contained in this Agreement by signing an additional Consent of
Spouse in the form of Exhibit A.

      7.    Legends. All certificates representing any shares of Stock subject
to the provisions of this Agreement shall have endorsed thereon the following
legends:

            (a)   "THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO A
REPURCHASE OPTION AND RIGHT OF FIRST REFUSAL IN FAVOR OF THE COMPANY OR ITS
ASSIGNEE SET FORTH IN AN AGREEMENT BETWEEN THE COMPANY AND THE REGISTERED
HOLDER, OR HIS OR HER PREDECESSOR IN INTEREST, A COPY OF WHICH IS ON FILE AT
THE PRINCIPAL OFFICE OF THIS COMPANY.

            (b)   "THE SECURITIES EVIDENCED BY THIS CERTIFICATE HAVE NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD,
TRANSFERRED, ASSIGNED OR HYPOTHECATED UNLESS THERE IS AN EFFECTIVE REGISTRATION
STATEMENT UNDER SUCH ACT COVERING SUCH SECURITIES, THE SALE MADE IN ACCORDANCE
WITH RULE 144 UNDER THE ACT, OR THE COMPANY RECEIVES AN OPINION OF COUNSEL FOR
THE HOLDER OF THESE SECURITIES REASONABLY SATISFACTORY TO THE COMPANY, STATING
THAT SUCH SALE, TRANSFER, ASSIGNMENT OR HYPOTHECATION IS EXEMPT FROM THE
REGISTRATION AND PROSPECTUS DELIVERY REQUIREMENTS OF SUCH ACT."

            (c)   Any legend required to be placed thereon by the federal or
state securities authorities.

      8.    Warranties and Representations. In connection with the proposed
purchase of the Stock, the Founder hereby agrees, represents and warrants as
follows:



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          (a)  The Founder is purchasing the Stock solely for his own account
for investment and not with a view to, or for resale in connection with, any
distribution thereof within the meaning of the Securities Act of 1933 as
amended (the "Act"). The Founder further represents that he or she does not have
any present intention of selling, offering to sell or otherwise disposing of or
distributing the Stock or any portion thereof; and that the entire legal and
beneficial interest of the Stock he or she is purchasing is being purchased for,
and will be held for the account of, the Founder only and neither in whole nor
in part for any other person.

          (b)  The Founder is aware of the Company's business affairs and
financial condition and has acquired sufficient information about the Company
to reach an informed and knowledgeable decision to acquire the Stock. The
Founder further represents and warrants that he or she has discussed the
Company and its plans, operations and financial condition with its officers,
has received all such information as he or she deems necessary and appropriate
to enable him or her to evaluate the financial risk inherent in making an
investment in the Stock and has received satisfactory and complete information
concerning the business and financial condition of the Company in response to
all inquiries in respect thereof.

          (c)  The Founder realizes that his or her purchase of the Stock will
be a highly speculative investment, and he is able, without impairing his
financial condition, to hold the Stock for an indefinite period of time and to
suffer a complete loss on his investment.

          (d)  The Company has disclosed to the Founder that:

               (i)  The sale of the Stock has not been registered under the
Act, and the Stock must be held indefinitely unless a transfer of it is
subsequently registered under the Act or an exemption from such registration is
available, and that the Company is under no obligation to register the Stock;

               (ii) The Company will make a notation in its records of the
aforementioned restrictions on transfer and legends.

     9.   Escrow. As security for his faithful performance of the terms of this
Agreement and to ensure the availability for delivery of the Stock upon
exercise of the Unvested Share Repurchase Option and the Right of First
Refusal herein provided for, the Founder agrees to deliver to and deposit with
Gray Cary Ware & Freidenrich, a Professional Corporation (the "Escrow Agent"),
as Escrow Agent in this transaction, two Stock Assignments duly endorsed (with
date and number of shares blank) in the form attached hereto as Exhibit B,
together with the certificate or certificates evidencing the Stock. Such
documents shall be held by the Escrow Agent pursuant to the Joint Escrow
Instructions of the Company and the Founder set forth in Exhibit C attached
hereto and incorporated by this reference, which instructions shall also be
delivered to the Escrow Agent at the closing hereunder.

     10.  Transfers in Violation of Agreement. The Company shall not be
required (i) to transfer on its books any shares of Stock of the Company which
shall have been sold or transferred in violation of any of the provisions set
forth in this Agreement or (ii) to treat as owner of such shares or to accord
the right to vote as such owner or to pay dividends to any transferee to whom
such shares shall have been so transferred.

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     11.  Rights as Shareholder.  Subject to the provisions of this Agreement,
the Founder shall exercise all rights and privileges of a shareholder of the
Company with respect to the Stock deposited in escrow.

     12.  Further Instruments.  The parties agree to execute such further
instruments and to take such further action as may reasonably be necessary to
carry out the intent of this Agreement.

     13.  "Market Stand-Off" Agreement.  Founder hereby agrees that in
connection with the IPO, during the period of duration (not to exceed 180 days)
specified by the Company and an underwriter of common stock of the Company
following the effective date of the registration statement of the Company filed
under the Securities Act with respect to the IPO, he shall not, to the extent
requested by the Company and such underwriter, directly or indirectly sell,
offer to sell, contract to sell (including, without limitation, any short sale),
grant any option to purchase, pledge or otherwise transfer or dispose of (other
than to donees who agree to be similarly bound) any securities of the Company
held by him at any time during such period except common stock included in such
registration. Founder agrees to the terms of any form of such a stand-off
agreement as approved by the company or the underwriter of the IPO.

     14.  Notice.  Any notice required or permitted hereunder shall be given in
writing and shall be deemed effectively given upon personal delivery, upon
deposit in the United States Post Office, by registered or certified mail with
postage and fees prepaid, or upon delivery to an overnight courier service
addressed to the other party at the address hereinafter shown below his
signature or at such other address as such party may designate by ten (10)
days' advance written notice to the other party.

     15.  Successors and Assigns.  This Agreement shall inure to the benefit
of, and be binding upon, the successors and assigns of each party, including,
without limitation, in the case of the Founder, Founder's heirs, executors,
administrators, successors and assigns.

     16.  Entire Agreement; Amendments.  This Agreement, together with the
Exhibits hereto, shall be construed under the laws of the State of California
(as it applies to agreements between California residents, entered into and to
be performed entirely within California), and constitutes the entire agreement
of the parties with respect to the subject matter hereof superseding all prior
written or oral agreements, and no amendment or addition hereto shall be deemed
effective unless agreed to in writing by the parties.

     17.  Right to Specific Performance.  The Founder agrees that the Company
shall be entitled to a decree of specific performance of the terms hereof or an
injunction restraining violation of this Agreement, said right to be in
addition to any other remedies available to the Company.

     18.  Separability.  If any provision of this Agreement is held by a court
of competent jurisdiction to be invalid, void or unenforceable, the remaining
provisions shall nevertheless continue in full force and effect without being
impaired or invalidated in any way and shall be construed in accordance with the
purposes and tenor and effect of this Agreement.


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<PAGE>   8
     19.  Tax Consequences and Tax Election Notification.

          (a)  The Founder understands that Section 83 of the Internal Revenue
Code of 1986, as amended (the "Code") taxes as ordinary income the difference
between the amount paid for the Stock and the fair market value of the Stock as
of the date any restrictions on the Stock lapse. In this context, "restriction"
means the right of the Company to buy back the stock pursuant to the Unvested
Share Repurchase Option. The Founder understands that he or she may elect to be
taxed at the time the stock is purchased rather than when and as the Unvested
Share Repurchase Option expires by filing an election under Section 83(b) of
the Code with the Internal Revenue Service (the "IRS") within 30 days from the
date of purchase. Even if the fair market value of the Stock equals the amount
paid for the Stock, the election must be made to avoid adverse tax consequences
in the future. The Founder understands that failure to make this filing timely
will result in the recognition of ordinary income by the Founder, as the
Unvested Share Repurchase Option lapses, on the difference between the purchase
price and the fair market value of the Stock at the time such restriction
lapses.

          (b)  The Founder understands that the purchase price of the Stock has
been set by the Board of Directors and that the Company believes this valuation
is a fair attempt to appraise it. The Founder understands, however, that if the
Founder files a Section 83(b) election, the Company can give no assurances that
the purchase price will be accepted as the fair market value of the Stock by
the IRS, and that the IRS could assert that the value of the Stock on the date
of purchase was substantially greater than the purchase price.

          If the IRS were to successfully argue in a tax determination that the
Stock had a value greater than the price paid by the Founder, and the Founder
has filed a Section 83(b) election, the additional value would constitute
ordinary income as of the date of its receipt. The additional taxes (and
interest) due would be payable by the Founder. There is no provision for the
Company to reimburse the Founder for any potential tax liability, and the
Founder assumes all responsibility for any such liability. If the additional
value attributed to the Stock was more than 25 percent of the Founder's gross
income for the year in which that value was taxable, the IRS would have six
years from the due date for filing of the Founder's the return (or the actual
filing date of the return if filed thereafter) within which to assess the
additional tax and interest.

          THE FOUNDER ACKNOWLEDGES THAT IT IS THE FOUNDER'S SOLE RESPONSIBILITY
AND NOT THE COMPANY'S RESPONSIBILITY TO FILE TIMELY THE ELECTION UNDER SECTION
83(b), EVEN IF THE FOUNDER REQUESTS THE COMPANY OR ITS REPRESENTATIVES TO MAKE
THIS FILING ON THE FOUNDER'S BEHALF. THE FOUNDER FURTHER UNDERSTANDS THAT ANY
PURPORTED ELECTION PURSUANT TO SECTION 83(b) MUST COMPLY WITH THE PROVISIONS OF
TREASURY REGULATION SECTION 1.83-2. FOUNDER ACKNOWLEDGES THAT HE HAS BEEN
ADVISED BY THE COMPANY TO SEEK THE ASSISTANCE OF A TAX ADVISOR IN THIS MATTER.

          (c)  The Founder shall notify the Company in writing if Founder files
an election pursuant to Section 83(b) of the Code. The Company intends, in the
event it does not receive from Founder evidence of such filing, to claim a tax
deduction for any amount which would be taxable to Founder in the absence of
such an election.


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<PAGE>   9

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the day and year first above written.

"FOUNDER"                                    "COMPANY"

Thomas Obenhuber                             MachOne Communications, Inc.

/s/ THOMAS OBENHUBER                         By:  /s/ PETER D. OLSON
----------------------------                    -------------------------------
Thomas Obenhuber

Address:  112 Wisconsin St.                  Title:  President
        --------------------                        ----------------------------

 San Francisco, CA 94107
----------------------------





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<PAGE>   10


                                   EXHIBIT A

                               CONSENT OF SPOUSE


     I, __________________, spouse of ________________, acknowledge that I have
read the Founder Stock Purchase Agreement dated as of ____________, 1997, to
which this Consent is attached as Exhibit A (the "Agreement") and that I know
its contents. I am aware that by its provisions the Company has the option to
purchase certain shares of Stock of the Company which my spouse owns pursuant to
the Agreement including any interest I might have therein, upon termination of
his employment under circumstances set forth in the Agreement, and that certain
other restrictions are imposed upon the sale or other disposition of the Stock
during my spouse's lifetime and in the event of his death.

     I agree that my interest, if any, in the Stock subject to the Agreement
shall be bound by the Agreement and further understand and agree that any
community property interest I may have in the Stock shall be similarly bound by
the Agreement.

     Signed and Dated:
                       ---------------------------------------



<PAGE>   11


                                   EXHIBIT B

                      ASSIGNMENT SEPARATE FROM CERTIFICATE


     FOR VALUE RECEIVED, ___________________ hereby sells, assigns and
transfers unto _______________   ____________ (________) shares of the Common
Stock of MachOne Communications, Inc., a California corporation, standing in
the undersigned's name on the books of said corporation represented by
Certificate No. ____ herewith, and do hereby irrevocably constitute and appoint
______________________ attorney to transfer the said stock on the books of the
said corporation with full power of substitution in the premises.


     Date:  December 23, 1997               By:  /s/ THOMAS OBENHUBER
                                                --------------------------------
                                                 Thomas Obenhuber






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