<PAGE>
                                                                    EXHIBIT 4.12

THIS CONVERTIBLE PROMISSORY NOTE AND THE SHARES ISSUABLE CONVERSION HEREOF HAVE
NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE
"SECURITIES ACT"), OR ANY STATE SECURITIES LAW. THE SECURITIES REPRESENTED
HEREBY MAY NOT BE OFFERED, SOLD OR OTHERWISE TRANSFERRED IN THE ABSENCE OF AN
EFFECTIVE REGISTRATION STATEMENT FOR SAID SECURITIES UNDER THE SECURITIES ACT
AND ANY OTHER APPLICABLE STATE SECURITIES LAWS OR RULES, UNLESS EXEMPTIONS FROM
THE REGISTRATION REQUIREMENTS OF SAID LAWS ARE AVAILABLE AND SAID OFFER, SALE OR
TRANSFER IS MADE PURSUANT TO AND IN STRICT COMPLIANCE WITH THE TERMS AND
CONDITIONS OF SAID EXEMPTIONS.

                                 WIRELESS, INC.
                           CONVERTIBLE PROMISSORY NOTE

$1,000,000.00                                                    August 17, 1999

     Wireless, Inc., a California corporation (the "Company"), hereby promises
to pay to AMT Capital, Ltd., a Texas limited partnership or its successors and
assigns (the "Payee"), the principal sum of ONE MILLION AND NO/100 DOLLARS
($1,000,000.00) on September 1, 2000, together with interest accrued thereon as
hereinafter provided.

     This Note is issued by the Company pursuant to the terms and provisions of
that certain Note Purchase Agreement dated as of August 17, 1999 by and between
the Company and Payee (the "Purchase Agreement"). This Note shall be deemed to
be a "Note" as that term is used in the Purchase Agreement. The Company agrees
that this Note is entitled to all of the benefits provided in the Purchase
Agreement including, without limitation, all representations, warranties and
covenants of the Company contained therein and all remedies provided therein.
Reference herein to the Purchase Agreement shall not affect or impair the
absolute and unconditional obligation of the Company to pay this Note when due.
Capitalized terms that are not defined herein shall have such meanings as are
assigned to them in the Purchase Agreement.

     The unpaid principal balance of this Note shall bear interest at a rate
equal to seventeen percent (17%) per annum. Accrued interest shall be calculated
on the basis of a 365-day year. Interest accrued hereon shall be due and payable
on the first day of each December, March, June and September, beginning December
1, 1999, until the entire unpaid principal amount of this Note is repaid to
Payee.

     Notwithstanding anything in this Note to the contrary, in the event the
Company obtains, no later than fourteen (14) days following the date hereof, the
final, full, irrevocable and unconditional waiver of all preemptive rights,
rights of first refusal, and similar rights regarding the issuance by the
Company to Payee of this Note and any and all capital stock upon the conversion
of this Note as provided herein (the "Waivers"), then thereafter (i) this Note
shall mature on September 1, 2004 instead of on September 1, 2000, and (ii) the
unpaid principal balance of this Note shall bear interest at a rate equal to ten
percent (10%) per annum.

     From and after the maturity date of this Note, any outstanding principal of
and interest on this Note shall bear interest at a rate equal to the maximum
rate allowed by applicable law.


<PAGE>

Payments under this Note shall be made by wire transfer on the date due to such
bank account that Payee shall designate to the Company in writing.

     At any time after October 31, 1999, the Company shall have the right to
prepay, upon 30 prior days written notice to Payee, a portion or all of the
unpaid principal of and accrued and unpaid interest on this Note. During such 30
day notice period, Payee shall have the right to exchange all or any portion of
the principal of and interest on this Note for shares of the Company's capital
stock in accordance with the provisions below.

     Upon the occurrence of any default or event of default under the Purchase
Agreement, Payee shall have the option of declaring the principal balance hereof
and the interest accrued hereon to be immediately due and payable.

     Payee shall have the right to convert all or any portion of the unpaid
principal amount of and interest on this Note as follows:

     (a) CONVERSION PROCEDURES.

          (i) CONVERSION RIGHT. Subject to and upon compliance with the terms
     hereof, at any time after the date hereof, the holder of this Note may
     elect, by giving written notice of such election to the Company, to convert
     all or any portion of the principal amount of and interest on this Note
     into shares of fully paid and non-assessable shares of the class of the
     Company's preferred stock purchased by the investor(s) in the Company's
     arms' length equity financing, if any, most immediately following the date
     hereof (such financing being the "Next Financing" and such class of
     preferred stock being the "Preferred Stock"), which number of shares shall
     be determined by dividing such amount by the "Preferred Stock Conversion
     Price" (as determined below) in effect at the time of conversion. The
     conversion price payable for each share of Preferred Stock into which this
     Note is convertible shall, subject to further adjustment as provided
     herein, be equal to the lesser of (i) 125% of the per-share purchase price
     paid for the Preferred Stock in the Next Financing, or (ii) $3.75 (as
     applicable, such per share price, as adjusted, the "Preferred Stock
     Conversion Price").

          In the event (a) an Organic Change (as defined below) occurs prior to
     the Next Financing, or (b) the Next Financing does not occur prior to
     October 31, 1999, then in either event and subject to and upon compliance
     with the terms hereof the holder of this Note may elect (at any time after
     receiving notice of the Organic Change in accordance with the terms hereof
     or at any time after October 31, 1999 if the Next Financing has not then
     occurred), by giving written notice of such election to the Company, to
     convert all or any portion of the principal amount of and interest on this
     Note into shares of fully paid and non-assessable shares of the Company's
     common stock (the "Common Stock"), which number of shares shall be
     determined by dividing such amount by the "Common Stock Conversion Price"
     (as determined below) in effect at the time of conversion. The conversion
     price payable for each share of Common Stock into which this Note is
     convertible shall, subject to further adjustment as provided herein, be
     equal to $1.25 (as adjusted, the "Common Stock Conversion Price" and unless
     otherwise specifically


                                       2
<PAGE>

     referenced, the Preferred Stock Conversion Price and the Common Stock
     Conversion Price are collectively referred to herein as the "Conversion
     Price").

          If only a portion of this Note is converted to capital stock, the
     Company shall return this Note to the Holder with a notation of the
     remaining outstanding principal of this Note or, at the request of the
     Holder of this Note, shall issue and deliver to such Holder, a replacement
     convertible promissory note for the remaining outstanding balance hereof.

          (ii) PREPAYMENT/CONVERSION RIGHT. In the event the Company notifies
     the Payee that the Company has elected to exercise its right to prepay this
     Note as set forth above, the Payee may elect to convert this Note into
     shares of capital stock pursuant hereto at any time prior to the actual
     tender of such payment by the Company in accordance with the terms hereof,
     and the conversion right granted to Payee pursuant hereto shall only
     terminate upon the actual tender for payment by the Company of the
     prepayment price of this Note to Payee in accordance with the terms hereof.
     In the event Payee elects to convert all or a portion of this Note after
     the Company elects to prepay this Note, but before payment of this Note is
     tendered in accordance with the terms hereof, the Company's prepayment
     right with respect to the amount designated to be converted shall terminate
     immediately upon Payee's notice of its election to convert, and the Note
     (or appropriate portion thereof) shall be converted into shares of capital
     stock in accordance with the terms hereof.

          (iii) DELIVERY OF CERTIFICATES. As soon as possible after conversion
     has been effected (but in any event within five business days), the Company
     will deliver to the converting holder a certificate or certificates
     representing the number of shares of capital stock issuable by reason of
     such conversion registered in such name or names and such denomination or
     denominations as the converting holder has specified.

          (iv) FURTHER ASSURANCES. The issuance of certificates for shares of
     capital stock upon conversion of this Note will be made without charge to
     the holder of this Note for any issuance tax in respect thereof or other
     cost incurred by the Company in connection with such conversion and the
     related issuance of shares of stock. Upon conversion of this Note, the
     Company will take all such actions as are necessary in order to ensure that
     the shares issuable with respect to such conversion will be validly issued,
     fully paid and nonassessable.

          (v) BOOKS TO BE KEPT OPEN. The Company will not close its books
     against the transfer of this Note or of capital stock issued or issuable
     upon conversion of this Note in any manner which interferes with the timely
     conversion of this Note.

          (vi) FRACTIONAL SHARES. If any fractional interest in a share would be
     deliverable upon any conversion of this Note, the Company, in lieu of
     delivering the fractional share therefor, will, pay an amount to the holder
     thereof equal to such fractional interest multiplied by the applicable
     Conversion Price as of the date of conversion.


                                       3
<PAGE>

     (b) ADJUSTMENTS TO CONVERSION PRICE AND NUMBER OF SHARES. The Conversion
Price and the number of shares into which this Note is convertible shall be
subject to adjustment as follows:

          (i) ISSUANCE OF ADDITIONAL SHARES.

               (A) If the Company issues or sells shares of Preferred Stock, or
          in any manner grants any warrants, options or other rights
          (collectively, "Preferred Options") to purchase shares of Preferred
          Stock or any other securities convertible into or exchangeable for
          Preferred Stock ("Preferred Convertible Securities"), which entitles
          the subscriber, or the holder of such Preferred Option or Preferred
          Convertible Security, to purchase any share of Preferred Stock at less
          than the then current Preferred Stock Conversion Price (if then
          applicable), then the Preferred Stock Conversion Price in effect
          immediately prior to such action by the Company shall be adjusted by
          being multiplied by the fraction obtained:

               by dividing

               (X), which is the numerator obtained by adding (A) the total
               number of issued and outstanding shares of Preferred Stock
               immediately prior to the effectiveness of such action by the
               Company, plus (B) the number of shares of Preferred Stock that
               could have been acquired, at the Preferred Stock Conversion Price
               in effect immediately prior thereto, with the consideration, if
               any, received or deemed received by the Company in exchange for
               such action,

               by

               (Y), which is the denominator that equals the sum of the actual
               total number of issued and outstanding shares, plus the number of
               shares in a Preferred Deemed Issue (defined below), of Preferred
               Stock immediately after such effectiveness.

               In case at any time the Company shall grant or issue any
          Preferred Options or Preferred Convertible Securities, whether or not
          such Preferred Options or the rights to convert or exchange any such
          Preferred Convertible Securities are immediately exercisable, and the
          price per share for which Preferred Stock is issuable upon the
          exercise of such Preferred Options or upon the conversion or exchange
          of such Preferred Convertible Securities (determined by dividing (x)
          the total amount, if any, received or receivable by the Company as
          consideration for the issue or grant of such Preferred Options and
          Preferred Convertible Securities, plus the minimum aggregate amount of
          additional consideration payable to the Company upon the exercise of
          such Preferred Options and the conversion or exchange of such
          Preferred Convertible Securities, including, in the case of any such
          Preferred Options which relate to Preferred Convertible Securities,
          the minimum aggregate amount of additional consideration, if any,
          payable to the Company upon the conversion or exchange of such
          Preferred Convertible


                                       4
<PAGE>

          Securities, by (y) the total maximum number of shares of Preferred
          Stock issuable upon the exercise of such Preferred Options and upon
          the conversion or exchange of all such Preferred Convertible
          Securities) shall be less than the Preferred Stock Conversion Price in
          effect immediately prior to the time of the granting of such Preferred
          Options or Preferred Convertible Securities, then the total maximum
          number of shares of Preferred Stock issuable upon the exercise of such
          Preferred Options and upon the conversion or exchange of the total
          maximum amount of such Preferred Convertible Securities (including
          those issuable upon the exercise of Preferred Options) shall, as of
          the date of granting of such Preferred Options and Preferred
          Convertible Securities, be deemed to be outstanding and to have been
          issued and sold for such price per share (a "Preferred Deemed Issue")
          and the provisions of this paragraph (i) shall apply accordingly.

               (B) If the Company issues or sells shares of Common Stock, or in
          any manner grants any warrants, options or other rights (collectively,
          "Common Options") to purchase shares of Common Stock or any other
          securities convertible into or exchangeable for Common Stock ("Common
          Convertible Securities"), which entitles the subscriber, or the holder
          of such Common Option or Common Convertible Security, to purchase any
          share of Common Stock at less than the then current Common Stock
          Conversion Price (if then applicable), then the Common Stock
          Conversion Price in effect immediately prior to such action by the
          Company shall be adjusted by being multiplied by the fraction
          obtained:

               by dividing

               (X), which is the numerator obtained by adding (A) the total
               number of issued and outstanding shares of Common Stock
               immediately prior to the effectiveness of such action by the
               Company, plus (B) the number of shares of Common Stock that could
               have been acquired, at the Common Stock Conversion Price in
               effect immediately prior thereto, with the consideration, if any,
               received or deemed received by the Company in exchange for such
               action,

               by

               (Y), which is the denominator that equals the sum of the actual
               total number of issued and outstanding shares, plus the number of
               shares in a Common Deemed Issue (defined below), of Common Stock
               immediately after such effectiveness.

               In case at any time the Company shall grant or issue any Common
          Options or Common Convertible Securities, whether or not such Common
          Options or the rights to convert or exchange any such Common
          Convertible Securities are immediately exercisable, and the price per
          share for which Common Stock is issuable upon the exercise of such
          Common Options or upon the conversion or exchange of such Common
          Convertible Securities (determined by dividing (x) the


                                       5
<PAGE>


          total amount, if any, received or receivable by the Company as
          consideration for the issue or grant of such Common Options and Common
          Convertible Securities, plus the minimum aggregate amount of
          additional consideration payable to the Company upon the exercise of
          such Common Options and the conversion or exchange of such Common
          Convertible Securities, including, in the case of any such Common
          Options which relate to Common Convertible Securities, the minimum
          aggregate amount of additional consideration, if any, payable to the
          Company upon the conversion or exchange of such Common Convertible
          Securities, by (y) the total maximum number of shares of Common Stock
          issuable upon the exercise of such Common Options and upon the
          conversion or exchange of all such Common Convertible Securities)
          shall be less than the Common Stock Conversion Price in effect
          immediately prior to the time of the granting of such Common Options
          or Common Convertible Securities, then the total maximum number of
          shares of Common Stock issuable upon the exercise of such Common
          Options and upon the conversion or exchange of the total maximum
          amount of such Common Convertible Securities (including those issuable
          upon the exercise of Common Options) shall, as of the date of granting
          of such Common Options and Common Convertible Securities, be deemed to
          be outstanding and to have been issued and sold for such price per
          share (a "Common Deemed Issue") and the provisions of this paragraph
          (i) shall apply accordingly.

               Notwithstanding anything in this Note to the contrary, this
          paragraph (b)(i)(B) shall not be applicable to any of the following:

               (1)  shares of Common Stock issued upon conversion of any of the
                    Company's preferred stock;

               (2)  shares of Common Stock issued to officers, directors,
                    employees and consultants or other service providers of the
                    Company pursuant to a stock grant, stock option plan or
                    purchase plan or other employee stock incentive program or
                    agreement approved by the Company's Board of Directors;

               (3)  shares of capital stock issued or issuable in connection
                    with a bona fide equipment lease or bank financing
                    transaction approved by the Company's Board of Directors,
                    including without limitation shares issued upon the exercise
                    of warrants issued in connection with such transactions; or

               (4)  shares of capital stock issued or issuable in connection
                    with a merger or acquisition transaction.

          (ii) TREATMENT OF EXPIRED OPTIONS AND UNEXERCISED CONVERTIBLE
     SECURITIES. Upon the expiration of any Preferred Option or Common Option or
     the termination of any right to convert or exchange any Preferred
     Convertible Security or Common Convertible Security without the exercise or
     conversion of any such option, convertible security or


                                       6
<PAGE>


     right, the Preferred Conversion Price or Common Conversion Price then in
     effect and the number of shares into which this Note is then convertible
     will be adjusted to the Preferred Conversion Price or Common Conversion
     Price, as applicable, and number of shares which would have been in effect
     at the time of such expiration or termination had such option or
     convertible security, to the extent outstanding immediately prior to such
     expiration or termination, never been issued.

          (iii) TREASURY SHARES. The number of shares of capital stock
     outstanding at any given time does not include shares owned or held by or
     for the account of the Company, and the disposition of any shares so owned
     or held will be considered an issue or sale of capital stock.

          (iv) RECORD DATE. If the Company takes a record of the holders of
     capital stock for the purpose of entitling them (A) to receive a dividend
     or other distribution payable in capital stock, options or in convertible
     securities or (B) to subscribe for or purchase capital stock, options or
     convertible securities, then such record date will be deemed to be the date
     of the issue or sale of the shares of capital stock deemed to have been
     issued or sold upon the declaration of such dividend or upon the making of
     such other distribution or the date of the granting of such right of
     subscription or purchase, as the case may be.

          (v) SUBDIVISION OR COMBINATION OF CAPITAL STOCK.

               If the Company at any time after the date hereof subdivides (by
          any stock split, stock dividend, recapitalization or otherwise) its
          outstanding shares of Preferred Stock into a greater number of shares,
          the Preferred Stock Conversion Price in effect immediately prior to
          such subdivision will be proportionately reduced and the number of
          shares of Preferred Stock into which this Note is then convertible
          shall be proportionately increased, and if the Company at any time
          combines (by reverse stock split or otherwise) its outstanding shares
          of Preferred Stock into a smaller number of shares, the Preferred
          Stock Conversion Price in effect immediately prior to such combination
          will be proportionately increased and the number of shares of
          Preferred Stock into which this Note is then convertible shall be
          proportionately decreased.

               If the Company at any time after the date hereof subdivides (by
          any stock split, stock dividend, recapitalization or otherwise) its
          outstanding shares of Common Stock into a greater number of shares,
          the Common Stock Conversion Price in effect immediately prior to such
          subdivision will be proportionately reduced and the number of shares
          of Common Stock into which this Note is then convertible shall be
          proportionately increased, and if the Company at any time combines (by
          reverse stock split or otherwise) its outstanding shares of Common
          Stock into a smaller number of shares, the Common Stock Conversion
          Price in effect immediately prior to such combination will be
          proportionately increased and the number of shares of Common Stock
          into which this Note is then convertible shall be proportionately
          decreased.


                                       7
<PAGE>


          (vi) REORGANIZATION, RECLASSIFICATION, CONSOLIDATION, MERGER OR SALE.
     Any capital reorganization, reclassification, consolidation, merger or sale
     of all or substantially all of the Company's assets to another person which
     is effected in such a way that holders of capital stock are entitled to
     receive (either directly or upon subsequent liquidation) stock, securities
     or assets with respect to or in exchange for capital stock is referred to
     herein as an "Organic Change." Prior to the consummation of any Organic
     Change, the Company will make appropriate provisions (in form and substance
     satisfactory to the holder of this Note) to ensure that such holder will
     thereafter have the right to acquire and receive, in lieu of or in addition
     to the shares of capital stock that immediately prior thereto are
     acquirable and receivable upon the conversion of this Note, such shares of
     stock, securities or assets as such holder would have received in
     connection with such Organic Change if such holder had fully converted this
     Note immediately prior to such Organic Change. The Company will not effect
     any such consolidation, merger or sale, unless prior to the consummation
     thereof, the successor (if other than the Company) resulting from
     consolidation or merger or the party purchasing such assets assumes by
     written instrument (in form reasonably satisfactory to the holder hereof),
     the obligation to deliver to such holder such shares of stock, securities
     or assets as, in accordance with the foregoing provisions, such holder may
     be entitled to acquire.

          (vii) CERTAIN EVENTS. If any event occurs of the type contemplated by
     the provisions of this section (b) but not expressly provided for by such
     provisions, then the Company's Board of Directors will make an appropriate
     adjustment in the Preferred Stock Conversion Price or Common Stock
     Conversion Price, as applicable, and the number of shares for which this
     Note is convertible so as to protect the rights of the holder hereof;
     provided, that no such adjustment will increase such conversion price as
     otherwise determined pursuant hereto or decrease the number of shares of
     capital stock issuable upon conversion hereof.

          (viii) NOTICES.

               (A) Immediately upon any adjustment of the Conversion Price or
     the number of shares for which this Note is convertible, the Company will
     give written notice thereof to the holder hereof.

               (B) The Company will give written notice to the holder hereof at
     least 10 days prior to the date on which the Company closes its books or
     declares a record date (1) with respect to any dividend or distribution
     upon (or any subdivision, combination or other change in the outstanding
     number of shares of) any class of the Company's capital stock, (2) with
     respect to any pro rata subscription offer to holders of any class of the
     Company's capital stock or (3) for determining rights to vote with respect
     to any Organic Change, dissolution or liquidation.


                                       8
<PAGE>


               (C) The Company also give written notice to the holder hereof at
     least 30 days prior to the date on which the Next Financing or any Organic
     Change will take place.

     The invalidity, or unenforceability in particular circumstances, of any
provision of this Note shall not extend beyond such provision or such
circumstances and no other provision of this Note shall be affected thereby. It
is expressly stipulated and agreed to be the intent of the Company and Payee at
all times to comply with applicable state law governing the maximum rate or
amount of interest payable on or in connection with this Note (or applicable
United States federal law to the extent that it permits Payee to contract for,
charge, take, reserve or receive a greater amount of interest than under state
law). If the applicable law is ever judicially interpreted so as to render
usurious any amount called for under this Note or under any of the other
documents evidencing, securing or relating to this Note or any part thereof
(collectively, the "Note Documents"), or contracted for, charged, taken,
reserved or received with respect to the indebtedness evidenced by this Note
(the "Loan"), or if any prepayment by the Company results in the Company having
paid any interest in excess of that permitted by law, then it is the Company's
and Payee's express intent that all excess amounts theretofore collected by
Payee be credited on the principal balance of this Note (or, if this Note has
been or would thereby be paid in full, refunded to the Company), and the
provisions of this Note and the other Note Documents immediately be deemed
reformed and the amounts thereafter collectible hereunder and thereunder
reduced, without the necessity of the execution of any new document, so as to
permit the recovery of the fullest amount called for hereunder and thereunder,
while complying in all respects with the applicable law and regulations. All
sums paid or agreed to be paid to Payee for the use, forbearance or detention of
the Loan shall, to the extent permitted by applicable law, be amortized,
prorated, allocated and spread throughout the full term of the Note until
payment in full so that the rate or amount of interest on account of the Note
does not exceed the applicable usury ceiling. Notwithstanding any provision
contained in this Note or in the Note Documents that permits the compounding of
interest, including without limitation any provision by which any of the accrued
interest is added to the principal amount of this Note, the total amount of
interest that the Company is obligated to pay and Payee is entitled to receive
with respect to this Note shall not exceed the amount calculated on a simple
(i.e., non-compounded) interest basis at the maximum allowable rate on principal
amounts actually advanced to or for the account of the Company.

     The Company and each surety, endorser, guarantor, and other party, if any,
now or hereafter liable for payment of any sums of money payable on this Note,
jointly and severally, waive presentment and demand for payment, notice of
intent to accelerate and notice of acceleration, protest and notice of protest
and nonpayment, and diligence in collecting or bringing suit against any party
liable hereon, and agree that their liability on this Note shall not be affected
by any


                                       9
<PAGE>

renewal or extension in time of payment hereof, by any indulgence, or by any
release, modification, or substitution of any security for the payment of this
Note, and hereby consent to any and all extensions, renewals, replacements,
waivers, releases, or exchanges affecting this Note and the taking, release,
modification, or substitution of any security, with or without notice and before
or after maturity.

     This Note shall be binding upon and inure to the benefit of the Company,
its successors and assigns, and shall inure to the benefit of the Payee, its
successors and permitted assigns, In the event this Note is placed in the hands
of an attorney for collection or suit is filed hereon or if proceedings are had
in bankruptcy, receivership, reorganization, or other legal or judicial
proceedings for the collection hereof, the Company hereby agrees to pay to the
holder of this Note reasonable attorneys' fees, and shall pay all additional
reasonable costs and expenses of collection and enforcement.

     THE SUBSTANTIVE LAWS OF THE STATE OF TEXAS SHALL GOVERN THE VALIDITY,
CONSTRUCTION, ENFORCEMENT, AND INTERPRETATION OF THIS NOTE WITHOUT REGARD TO
CONFLICT ON LAWS PROVISIONS.

     IN WITNESS WHEREOF, the Company has caused its duly authorized officer to
execute this Note as of the day and year first written above,

                                        WIRELESS, INC.


                                        By: /s/ WILLIAM E. GIBSON
                                           -------------------------------------
                                           Name: WILLIAM E. GIBSON
                                                --------------------------------
                                           Title: PRESIDENT
                                                 -------------------------------