<PAGE>

                                                                     EXHIBIT 4.5

      Neither this warrant nor the shares of stock issuable upon exercise hereof
      have been registered under the Securities Act of 1933, as amended (the
      "Act") or under the laws of any state. No sale, transfer or other
      disposition of this warrant or such shares may be effected without (i) an
      effective registration statement under such laws related thereto or (ii)
      an opinion of counsel for the holder, or other evidence, reasonably
      satisfactory to the Company, that such registration is not required.

                          OPEN PORT TECHNOLOGY, INC.

                       PREFERRED STOCK PURCHASE WARRANT
                       --------------------------------


Date of Issuance:  June 22, 1998  Certificate No. PW-1


      FOR VALUE RECEIVED, OPEN PORT TECHNOLOGY, INC., an Illinois corporation
(the "Company"), hereby grants to Third Coast Venture Lease Partners I, L.P.
("Third Coast") or its registered assigns (the "Registered Holder") the right to
purchase from the Company 9,876 shares of Warrant Stock at a price per share of
$2.025 (as adjusted from time to time hereunder, the "Exercise Price").  This
Warrant is being issued pursuant to the terms of the Warrant Purchase Agreement,
dated as of June 22, 1998 (the "Purchase Agreement"), between the Company and
Third Coast.  Certain capitalized terms used herein are defined in Section 5
hereof.  The amount and kind of securities obtainable pursuant to the rights
granted hereunder and the purchase price for such securities are subject to
adjustment pursuant to the provisions contained in this Warrant.

      This Warrant is subject to the following provisions:

Section 1.  Exercise of Warrant.
            -------------------

1.A.   Exercise Period.  The Registered Holder may exercise, in whole or in
       ---------------
part, the purchase rights represented by this Warrant at any time and from time
to time after the date of issuance hereof to and including the tenth anniversary
of the Commencement Date (as defined in the Equipment Financing) of the last
Schedule under the Equipment Financing with Third Coast (the "Exercise Period").
The Company shall give the Registered Holder written notice of the expiration of
the Exercise Period at least 30 days but not more than 90 days prior to the end
of the Exercise Period.  For purposes
<PAGE>

of this Warrant, "Trigger Date" shall mean the date upon which the aggregate
amount of funds provided to the Company by Third Coast under the Equipment
Financing exceeds $500,000.

1.B.   Exercise Procedure.
       ------------------

(i)   This Warrant shall be deemed to have been exercised when the Company has
received all of the following items (the "Exercise Time"):

      (a)  a completed Exercise Agreement, as described in paragraph 1C below,
      executed by the Person exercising all or part of the purchase rights
      represented by this Warrant (the "Purchaser");

      (b)  this Warrant;

      (c)  if this Warrant is not registered in the name of the Purchaser, an
      Assignment or Assignments in the form set forth in Exhibit II hereto
                                                         ----------
      evidencing the assignment of this Warrant to the Purchaser, in which case
      the Registered Holder shall have complied with the provisions set forth in
      Section 7 hereof; and

      (d)  either (1) a cashier's check payable to the Company, or a wire
      transfer to the account designated by the Company, in an amount equal to
      the product of the Exercise Price multiplied by the number of shares of
      Warrant Stock being purchased upon such exercise (the "Aggregate Exercise
      Price"), (2) the surrender to the Company of debt or equity securities of
      the Company having a fair market value (determined by the Board of
      Directors in their reasonable good faith judgment) equal to the Aggregate
      Exercise Price of the Warrant Stock being purchased upon such exercise
      (provided that for purposes of this subparagraph, the fair market value of
      any note or other debt security or any preferred stock shall be deemed to
      be equal to the aggregate outstanding principal amount or liquidation
      value thereof plus all accrued and unpaid interest thereon or accrued or
      declared and unpaid dividends thereon) or (3) a written notice to the
      Company that the Purchaser is exercising the Warrant (or a portion
      thereof) by authorizing the Company to withhold from issuance a number of
      shares of Warrant Stock issuable upon such exercise of the Warrant which
      when multiplied by the Market Price of the Warrant Stock (assuming
      conversion thereof)is equal to the Aggregate Exercise Price (and such
      withheld shares shall no longer be issuable under this Warrant).

                                      -2-
<PAGE>

(ii) Certificates for shares of Warrant Stock purchased upon exercise of this
Warrant shall be delivered by the Company to the Purchaser within five business
days after the date of the Exercise Time. Unless this Warrant has expired or all
of the purchase rights represented hereby have been exercised, the Company shall
prepare a new Warrant, substantially identical hereto, representing the rights
formerly represented by this Warrant which have not expired or been exercised
and shall, within such five-day period, deliver such new Warrant to the Person
designated for delivery in the Exercise Agreement.

(iii) The Warrant Stock issuable upon the exercise of this Warrant shall be
deemed to have been issued to the Purchaser at the Exercise Time, and the
Purchaser shall be deemed for all purposes to have become the record holder of
such Warrant Stock at the Exercise Time.

(iv) The issuance of certificates for shares of Warrant Stock upon exercise of
this Warrant shall be made without charge to the Registered Holder or the
Purchaser for any issuance tax in respect thereof or other cost incurred by the
Company in connection with such exercise and the related issuance of shares of
Warrant Stock. Each share of Warrant Stock issuable upon exercise of this
Warrant shall, upon payment of the Exercise Price therefor, be fully paid and
nonassessable and free from all liens and charges with respect to the issuance
thereof, excluding taxes based on the income of Purchaser.

(v) Assuming the satisfaction of all conditions for transfer, the Company
shall not close its books against the transfer of this Warrant or of any share
of Warrant Stock issued or issuable upon the exercise of this Warrant in any
manner which interferes with the timely exercise of this Warrant. The Company
shall from time to time take all such action as may be necessary to assure that
the par value per share of the unissued Warrant Stock acquirable upon exercise
of this Warrant is at all times equal to or less than the Exercise Price then in
effect.

(vi) The Company shall reasonably cooperate with any Registered Holder or
Purchaser required to make any governmental filings or obtain any governmental
approvals prior to or in connection with any exercise of this Warrant
(including, without limitation, making any filings required to be made by the
Company), except for registrations under the Act, the Securities Exchange Act of
1934 or under the laws of any state.

(vii) Notwithstanding any other provision hereof, if an exercise of any portion
of this Warrant is to be made in connection with a registered public offering or
the sale of the Company, the

                                      -3-
<PAGE>

exercise of any portion of this Warrant may, at the election of the holder
hereof, be conditioned upon the consummation of the public offering or the sale
of the Company in which case such exercise shall not be deemed to be effective
until the consummation of such transaction.

(viii) The Company shall at all times reserve and keep available out of its
authorized but unissued shares of Warrant Stock solely for the purpose of
issuance upon the exercise of the Warrant, such number of shares of Warrant
Stock issuable upon the exercise of this Warrant. The Company shall take all
such actions as may be necessary to assure that all such shares of Warrant Stock
may be so issued without violation of any applicable law or governmental
regulation or any requirements of any domestic securities exchange upon which
shares of Warrant Stock may be listed (except for official notice of issuance
which shall be immediately delivered by the Company upon each such issuance).
The Company shall not take any action which would cause the number of authorized
but unissued shares of Warrant Stock to be less than the number of such shares
required to be reserved hereunder for issuance upon exercise of this Warrant.

1.C.      Exercise Agreement.  Upon any exercise of this Warrant, the Exercise
          ------------------
Agreement shall be substantially in the form set forth in Exhibit I hereto,
                                                          ---------
except that if the shares of Warrant Stock are not to be issued in the name of
the Person in whose name this Warrant is registered, the Exercise Agreement
shall also state the name of the Person to whom the certificates for the shares
of Warrant Stock are to be issued, and if the number of shares of Warrant Stock
to be issued does not include all the shares of Warrant Stock purchasable
hereunder, it shall also state the name of the Person to whom a new Warrant for
the unexercised portion of the rights hereunder is to be delivered. Such
Exercise Agreement shall be dated the actual date of execution thereof.

1.D.      Conversion or Redemption of the Preferred Stock.  Notwithstanding
          -----------------------------------------------
any other provision of this Warrant, if all of the issued and outstanding shares
of the Company's Preferred Stock are (i) converted automatically into shares of
Common Stock pursuant to paragraph 4(b) of the Company's Amended and Restated
Articles of Incorporation (the "Automatic Conversion") or (ii) redeemed by the
Company pursuant to paragraph 5 of the Company's Amended and Restated Articles
of Incorporation (the "Complete Redemption"), this Warrant shall no longer be
exercisable into shares of Preferred Stock but immediately upon the consummation
of the Automatic Conversion or the Complete Redemption shall become exercisable
into a number of shares of Common Stock equal to the number of shares of Common
Stock issuable upon conversion of the shares of Preferred Stock issuable upon
exercise of this Warrant as

                                      -4-
<PAGE>

of the consummation of such event at an Exercise Price equal to the Series C
Conversion Price (as defined under the Company's Amended and Restated Articles
of Incorporation) in effect as of the consummation of such event. Following the
occurrence of the Automatic Conversion or the Complete Redemption, the
provisions of this Warrant shall continue to apply to the Warrant Stock which
shall then be Common Stock. In addition to the foregoing, if the Complete
Redemption has occurred, within 90 days thereafter and prior to any exercise of
this Warrant, the holder of this Warrant may elect to surrender this Warrant to
the Company for an amount payable in cash upon such surrender equal to the
aggregate Fair Market Value (as defined in the Company's Amended and Restated
Articles of Incorporation) of the Preferred Stock issuable upon exercise of this
Warrant as of the Complete Redemption reduced by the aggregate Exercise Price of
this Warrant at such time.

Section 2.  Adjustment of Exercise Price and Number of Shares.  The Exercise
            -------------------------------------------------
Price and the number of shares of Warrant Stock issuable upon exercise of this
Warrant shall be subject to adjustment from time to time as provided in this
Section 2.

2.A.           Subdivision or Combination of Warrant Stock.  If the Company at
               -------------------------------------------
any time subdivides (by any stock split, stock dividend, recapitalization or
otherwise) the Warrant Stock into a greater number of shares, the Exercise Price
in effect immediately prior to such subdivision shall be proportionately reduced
and the number of shares of Warrant Stock obtainable upon exercise of this
Warrant shall be proportionately increased, and if the Company at any time
combines (by reverse stock split or otherwise) the Warrant Stock into a smaller
number of shares, the Exercise Price in effect immediately prior to such
combination shall be proportionately increased and the number of shares of
Warrant Stock obtainable upon exercise of this Warrant shall be proportionately
decreased.

2.B.           Reorganization, Reclassification, Consolidation, Merger or Sale.
               ---------------------------------------------------------------
Any recapitalization, reorganization, reclassification, consolidation, merger,
sale of all or substantially all of the Company's assets or other transaction,
which in each case is effected in such a way that the holders of Warrant Stock
are entitled to receive (either directly or upon subsequent liquidation) stock,
securities or assets with respect to or in exchange for Warrant Stock is
referred to herein as "Organic Change." Prior to the consummation of any Organic
Change, the Company shall make appropriate provision to insure that the
Registered Holder shall thereafter have the right to acquire and receive, in
lieu of or addition to (as the case may be) the shares of Warrant Stock
immediately theretofore acquirable and receivable upon the exercise of such
holder's Warrant, such shares of stock, securities or assets as may be issued or
payable with respect to or in exchange

                                      -5-
<PAGE>

for the number of shares of Warrant Stock immediately theretofore acquirable and
receivable upon exercise of such holder's Warrant had such Organic Change not
taken place. In any such case, the Company shall make appropriate provision with
respect to such holders' rights and interests to insure that the provisions of
this Section 2 and Sections 3 and 4 hereof shall thereafter be applicable to
this Warrant. The Company shall not effect any such consolidation, merger or
sale, unless prior to the consummation thereof, the successor entity (if other
than the Company) resulting from consolidation or merger or the entity
purchasing such assets assumes by written instrument, the obligation to deliver
to each such holder such shares of stock, securities or assets as, in accordance
with the foregoing provisions, such holder may be entitled to acquire.

2.C.           Certain Events.  If any event occurs of the type contemplated by
               --------------
the provisions of this Section 2 but not expressly provided for by such
provisions, then the Company's board of directors shall make an appropriate
adjustment in the Exercise Price and the number of shares of Warrant Stock
obtainable upon exercise of this Warrant so as to protect the rights of the
holder of this Warrant; provided that no such adjustment shall increase the
Exercise Price or decrease the number of shares of Warrant Stock obtainable as
otherwise determined pursuant to this Section 2.

2.D.           Adjustments.  No adjustments in the Exercise Price need be made
               -----------
if such adjustment would result in a change in the Exercise Price of less than
$0.01.  Any adjustment of less than $0.01 which is not made shall be carried
forward and shall be made at the time of and together with any subsequent
adjustment which, on a cumulative basis, amounts to an adjustment of $0.01 or
more in the Exercise Price.

2.E.           Notices.
               -------

(i) Immediately upon any adjustment of the Exercise Price, the Company shall
give written notice thereof to the Registered Holder, setting forth in
reasonable detail, and certifying the calculation of, such adjustment.

(ii) The Company shall give written notice to the Registered Holder at least 20
days prior to the date on which the Company closes its books or takes a record
(A) with respect to any dividend or distribution upon the Warrant Stock or the
Common Stock, (B) with respect to any pro rata subscription offer to holders of
Warrant Stock or Common Stock or (C) for determining rights to vote with respect
to any Organic Change, dissolution or liquidation.

                                      -6-
<PAGE>

(iii) The Company shall also give written notice to the Registered Holder at
least 20 days prior to the date on which any Organic Change, dissolution or
liquidation shall take place.

Section 3.  Definitions.  The following terms have meanings set forth below:
            -----------

             "Common Stock" means, collectively, the Company's Common Stock
              ------------
and any capital stock of any class of the Company hereafter authorized which is
not limited to a fixed sum or percentage of par or stated value in respect to
the rights of the holders thereof to participate in dividends or in the
distribution of assets upon any liquidation, dissolution or winding up of the
Company.

             "Equipment Financing" has the meaning set forth in the Purchase
              -------------------
Agreement.

             "Market Price" means with respect to each share of Common Stock:
              ------------

     (i) if the exercise is in connection with an initial public offering, and
     if the Company's Registration Statement relating to such public offering
     has been declared effective by the SEC, then the initial "Price to Public"
     specified in the final prospectus with respect to the offering (the
     foregoing shall not be construed as giving the Warrantholder any right to
     participate in or require any such initial public offering);

     (ii) if this Warrant is exercised after, and not in connection with the
     Company's initial public offering, and:

          (a)  if traded on a securities exchange, the Market Price shall be
               deemed to be the average of the closing prices over a twenty-one
               (21) day period ending three days before the day the current
               Market Price of the securities is being determined; or

          (b)  if actively traded over-the-counter, the Market Price shall be
               deemed to be the average of the closing bid and asked prices
               quoted on the NASDAQ system (or similar system) over the twenty-
               one (21) day period ending three days before the day the current
               Market Price of the securities is being determined ;

     (iii) if at any time the Common Stock is not listed on any securities
     exchange or quoted in the NASDAQ

                                      -7-
<PAGE>

     System or the over-the-counter market, the current Market Price of Common
     Stock shall be the highest price per share which the Company could obtain
     from a willing buyer (not a current employee or director) for shares of
     Common Stock sold by the Company, from authorized but unissued shares, as
     determined in good faith by its Board of Directors, unless the Company
     shall become subject to a merger, acquisition or other consolidation
     pursuant to which the Company is not the surviving party, in which case the
     Market Price of Common Stock shall be deemed to be the value received by
     the holders of the Company's Common Stock on a common equivalent basis
     pursuant to such merger or acquisition.

          "Person" means an individual, a partnership, a joint venture, a
           ------
corporation, a limited liability company, a trust, an unincorporated
organization and a government or any department or agency thereof.

          "Preferred Stock" means the Company's Series C Convertible
           ---------------
Participating Preferred Stock, par value $.001 per share.

          "Warrant Stock" means the Company's Preferred Stock; provided that if
           -------------
there is a change such that the securities issuable upon exercise of the Warrant
are issued by an entity other than the Company or there is a change in the type
or class of securities so issuable, then the term "Warrant Stock" shall mean one
share of the security issuable upon exercise of the Warrant if such security is
issuable in shares, or shall mean the smallest unit in which such security is
issuable if such security is not issuable in shares.

          Other capitalized terms used in this Warrant but not defined herein
shall have the meanings set forth in the Purchase Agreement.

Section 4.  No Voting Rights; Limitations of Liability.  This Warrant shall not
            ------------------------------------------
entitle the holder hereof to any voting rights or other rights as a stockholder
of the Company. No provision hereof, in the absence of affirmative action by the
Registered Holder to purchase Warrant Stock, and no enumeration herein of the
rights or privileges of the Registered Holder, shall give rise to any liability
of such holder for the Exercise Price of Warrant Stock acquirable by exercise
hereof or as a stockholder of the Company.

Section 5.  Warrant Transferable.  Subject to the transfer conditions referred
            --------------------
to in the legend endorsed hereon, this Warrant and all rights hereunder are
transferable, in whole or in

                                      -8-
<PAGE>

part, without charge to the Registered Holder, upon surrender of this Warrant
with a properly executed Assignment (in the form of Exhibit II hereto) at the
                                                    ----------
principal office of the Company.

Section 6.  Warrant Exchangeable for Different Denominations.  This Warrant is
            ------------------------------------------------
exchangeable, upon the surrender hereof by the Registered Holder at the
principal office of the Company, for new Warrants of like tenor representing in
the aggregate the purchase rights hereunder, and each of such new Warrants shall
represent such portion of such rights as is designated by the Registered Holder
at the time of such surrender. The date the Company initially issues this
Warrant shall be deemed to be the "Date of Issuance" hereof regardless of the
number of times new certificates representing the unexpired and unexercised
rights formerly represented by this Warrant shall be issued.

Section 7.  Replacement.  Upon receipt of evidence reasonably satisfactory to
            -----------
the Company (a sworn, notarized affidavit of the Registered Holder shall be
satisfactory) of the ownership and the loss, theft, destruction or mutilation of
any certificate evidencing this Warrant, and in the case of any such loss, theft
or destruction, upon receipt of indemnity reasonably satisfactory to the
Company, or, in the case of any such mutilation upon surrender of such
certificate, the Company shall (at the Warrantholder's expense) execute and
deliver in lieu of such certificate a new certificate of like kind representing
the same rights represented by such lost, stolen, destroyed or mutilated
certificate and dated the date of such lost, stolen, destroyed or mutilated
certificate.

Section 8.  Notices.  Except as otherwise expressly provided herein, all
            -------
notices referred to in this Warrant shall be in writing and shall be delivered
personally, sent by reputable overnight courier service (charges prepaid) or
sent by registered or certified mail, return receipt requested, postage prepaid
and shall be deemed to have been given when so delivered, sent or deposited in
the U. S. Mail (i) to the Company, at its principal executive offices and (ii)
to the Registered Holder of this Warrant, at such holder's address as it appears
in the records of the Company (unless otherwise indicated by any such holder).

Section 9.  Amendment and Waiver.  Except as otherwise provided herein, the
            --------------------
provisions of this Warrant may be amended and the Company may take any action
herein prohibited, or omit to perform any act herein required to be performed by
it, only if the Company has obtained the written consent of the Registered
Holder.

Section 10.  Descriptive Headings; Governing Law.  The descriptive headings of
             -----------------------------------
the several Sections and paragraphs of this

                                 *     *     *

                                      -9-
<PAGE>

Warrant are inserted for convenience only and do not constitute a part of this
Warrant. The corporation laws of the State of Illinois shall govern all issues
concerning the relative rights of the Company and its stockholders. All other
questions concerning the construction, validity, enforcement and interpretation
of this Warrant shall be governed by the internal law of the State of Illinois
without giving effect to any choice of law or conflict of law provision or rule
(whether of the State of Illinois or any other jurisdictions) that would cause
the application of the laws of any jurisdictions other than the State of
Illinois.

          IN WITNESS WHEREOF, the Company has caused this Warrant to be signed
and attested by its duly authorized officers under its corporate seal and to be
dated the Date of Issuance hereof.


                                OPEN PORT TECHNOLOGY, INC.


                                By:  ____________________________

                                Its: ___________________________

                                      -10-
<PAGE>

        SCHEDULE OF THIRD COAST VENTURE LEASE PARTNERS 1, L.P. WARRANTS

--------------------------------------------------------------------------------
                                                                Number of Shares
 No.           Date of Warrant          Exercise Price          Subject to
                                                                Purchase
--------------------------------------------------------------------------------
 1.            6/22/98                  $2.03                   9,876
--------------------------------------------------------------------------------
 2.            6/22/98                  $2.03                   9,877
--------------------------------------------------------------------------------