<PAGE>

                                                                   EXHIBIT 10.12

THE SECURITIES REPRESENTED HEREBY 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 IS 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.


                                NEOPOINT, INC.
                   SERIES C PREFERRED STOCK PURCHASE WARRANT

     THIS CERTIFIES THAT, for value received, Siemens Aktiengesellschaft
("Holder") is entitled to purchase One Million Five Hundred Thousand (1,500,000)
shares of Series C Preferred Stock ("Warrant Shares") of NeoPoint, Inc., a
California corporation (the "Company"), at the Warrant Price (as defined in
subsection 1(j) below) of Six Dollars ($6.00), subject to adjustments and all
other terms and conditions set forth in this Warrant.

     1.   Definitions. As used herein, the following terms, unless the context
          -----------
otherwise requires, shall have the following meanings:

          (a)  "Act" shall mean the Securities Act of 1933, as amended, or any
successor federal statute, and the rules and regulations of the Commission
thereunder, all as the same shall be in effect at the time.

          (b)  "Commission" shall mean the Securities and Exchange Commission or
any other federal agency at the time administering the Act.

          (c)  "Common Stock" shall mean shares of the Company's presently or
subsequently authorized Common Stock, and any stock into which such Common Stock
may hereafter be exchanged.

          (d)  "Company" shall mean NeoPoint, Inc., a California corporation,
and any corporation which shall succeed to or assume the obligations of
NeoPoint, Inc. under this Warrant.

          (e)  "Date of Grant" shall mean December 3, 1999.

          (f)  "Exercise Date" shall mean the effective date of the delivery of
the Notice of Exercise pursuant to Sections 4 and 11 below.

          (g)  "Holder" shall mean Siemens Aktiengesellschaft or any other
person or entity who shall at the time be the registered holder of this Warrant.
<PAGE>

          (h)  "Series C Preferred Stock" shall mean shares of the Company's
Series C Preferred Stock, as described in the Company's Amended and Restated
Articles of Incorporation.

          (i)  "Shares" shall mean shares of the Company's Series C Preferred
Stock, as described in the Company's Articles of Incorporation .

          (j)  "Warrant Price" shall mean $6.00 per share.

     2.   Issuance of Warrant and Consideration Therefor. This Warrant is issued
          ----------------------------------------------
in consideration of the execution of that certain Series C Preferred Stock
Purchase Agreement dated October 7, 1999 and the purchase by Holder of the
Shares thereunder.

     3.   Term. The purchase right represented by this Warrant is exercisable
          ----
only during the period commencing upon the date hereof and ending on the earlier
of (i) January 31, 2001 or (ii) the closing of an initial public offering of the
Company's securities filed pursuant to a registration statement under the
Securities Act of 1933, as amended.

     4.   Exercise of Warrant.
          -------------------

          (a)  Exercise. This Warrant may be exercised, in whole or in part, by
               --------
the Holder hereof by surrender of this Warrant, with the form of subscription at
the end hereof duly executed by the Holder, to the Company at its principal
office, accompanied by payment, in cash or by certified or official bank check
payable to the order of the Company in the amount obtained by multiplying the
number of Warrant Shares for which this Warrant is being exercised by the
Warrant Price then in effect.

          (b)  Delivery of Certificate. In the event of any exercise of the
               -----------------------
purchase right represented by this Warrant, certificates for the Warrant Shares
so purchased shall be delivered to the Holder within thirty (30) days of
delivery of the notice of exercise (the "Notice of Exercise") in the form of
Exhibit A attached hereto and, unless this Warrant has been fully exercised or
---------
has expired, a new warrant representing the portion of the Warrant Shares with
respect to which this Warrant shall not then have been exercised shall also be
issued to the Holder within such thirty (30) day period.

          (c)  No Fractional Shares. No fractional shares shall be issued in
               --------------------
connection with any exercise hereunder, but in lieu of such fractional shares
the Company shall make a cash payment therefor upon the basis of the Fair Market
Value of a share of Series C Preferred Stock as of the Exercise Date.

          (d)  Company's Representations.
               -------------------------

               (i)  All Warrant Shares which may be issued upon the exercise
of the purchase right represented by this Warrant shall, upon issuance, be duly
authorized, validly issued, fully paid and nonassessable, and free of any liens
and encumbrances except for restrictions on transfer provided for herein or
under applicable federal and state securities laws. During the period within
which the purchase right represented by this Warrant may be exercised, the
Company shall at all times have authorized, and reserved for the purpose of
issuance upon

                                       2
<PAGE>

exercise of the purchase right represented by this Warrant, a sufficient number
of Shares to provide for the exercise of the purchase right represented by this
Warrant;

               (ii)  This Warrant has been duly authorized and executed by the
Company and is a valid and binding obligation of the Company enforceable in
accordance with its terms, subject to applicable bankruptcy, insolvency,
reorganization, moratorium or other laws of general application affecting the
enforcement of creditors' rights;

               (iii) The execution and delivery of this Warrant are not, and the
issuance of the Shares upon exercise of this Warrant in accordance with the
terms hereof will not be inconsistent with the Articles of Incorporation, as the
same may be amended from time to time, or Bylaws, do not and will not contravene
any law, governmental rule or regulation, judgment or order applicable to the
Company, and do not and will not conflict with or contravene any provision of,
or constitute a material default under, any material indenture, mortgage,
contract or other instrument of which the Company is a party or by which it is
bound or require the consent or approval of, the giving of notice to, the
registration or filing with or the taking of any action in respect of or by, any
federal, state or local government authority or agency (other than such
consents, approvals, notices, actions, filings, etc., as have already been
obtained or made, as the case may be).

     5.   Adjustment of Warrant Price and Number of Warrant Shares. The number
          --------------------------------------------------------
of securities issuable upon the exercise of this Warrant and the Warrant Price
shall be subject to adjustment from time to time upon the occurrence of certain
events, as follows:

          (a)  Adjustment for Dividends in Stock. In case at any time or from
               ---------------------------------
time to time the holders of Series C Preferred Stock of the Company (or any
shares of stock or other securities at the time receivable upon the exercise of
this Warrant) shall have received or, on or after the record date fixed for the
determination of eligible stockholders, shall have become entitled to receive,
without payment therefor, other or additional stock of the Company by way of
dividend then, and in each case, the Holder of this Warrant shall, upon the
exercise hereof, be entitled to receive, in addition to the number of Shares
receivable thereupon, and without payment of any additional consideration
therefor, the amount of such other or additional stock of the Company which such
Holder would hold on the date of such exercise had it been the holder of record
of Shares on the date hereof and had thereafter, during the period from the date
hereof to and including the date of such exercise, retained such shares and/or
all other additional stock receivable by it as aforesaid during such period,
giving effect to all adjustments called for during such period by subparagraphs
(b) and (c) of this Paragraph 5.

          (b)  Adjustment for Reclassification or Reorganization. In case of any
               -------------------------------------------------
reclassification or change of the outstanding securities of the Company or of
any reorganization of the Company, then and in each such case the Holder of this
Warrant, upon the exercise hereof at any time after the consummation of such
reclassification, change, or reorganization, shall be entitled to receive, in
lieu of or in addition to the stock or other securities and property receivable
upon the exercise hereof prior to such consummation, the stock or other
securities to which such Holder would have been entitled upon such consummation
if such Holder had exercised this Warrant immediately prior thereto, all subject
to further adjustment as provided in subparagraphs (a) and (c); in each such
case, the terms of this Paragraph 5 shall be applicable to the shares of

                                       3
<PAGE>

stock or other securities and property receivable upon the exercise of this
Warrant after such consummation.

          (c)  Stock Splits and Reverse Stock Splits. If the Company shall
               -------------------------------------
subdivide its outstanding shares of Series C Preferred Stock into a greater
number of shares, the Warrant Price in effect immediately prior to such
subdivision shall thereby be proportionately reduced and the number of Shares
receivable upon exercise of this Warrant shall thereby be proportionately
increased; and, conversely, if the outstanding number of shares of Common Stock
shall be combined into a smaller number of shares, the Warrant Price in effect
immediately prior to such combination shall thereby be proportionately increased
and the number of Shares receivable upon exercise of the Warrant shall be
proportionately decreased.

     6.   Notices of Record Date, Etc. In the event of (a) any taking by the
          ---------------------------
Company of a record of the holders of any class of securities for the purpose of
determining the holders thereof who are entitled to receive any dividend or
other distribution (the "Distribution"), (b) any capital reorganization or
reclassification of the stated capital of the Company or any consolidation or
merger of the Company with any other corporation or corporations (other than a
wholly-owned subsidiary), or the sale or distribution of all or substantially
all of the Company's property and assets (the "Reorganization Event"), or (c)
any proposed filing of a registration statement under the Act in connection with
a primary public offering of the Company's Common Stock (the "Registration
Event"), the Company will mail or cause to be mailed to the Holder a notice
specifying (i) the date of any such Distribution stating the amount and
character of such Distribution, (ii) the date on which any such Reorganization
Event or Registration Event is expected to become effective, and (iii) the time,
if any, that is to be fixed as to when the holders of record of the Company's
securities shall be entitled to exchange their shares of the Company's
securities for securities or other property deliverable upon such Reorganization
Event. Such notice shall be mailed at least thirty (30) days prior to the date
therein specified.

     7.   Compliance with Act; Transferability and Negotiability of Warrant;
          ------------------------------------------------------------------
Disposition of Shares.
---------------------

          (a)  Compliance with Act. The Holder, by acceptance hereof, agrees
               -------------------
that this Warrant and the Shares to be issued upon the exercise hereof are being
acquired solely for its own account and not as a nominee for any other party and
not with a view toward the resale or distribution thereof and that it will not
offer, sell or otherwise dispose of this Warrant or any Shares to be issued upon
the exercise hereof except under circumstances which will not result in a
violation of the Act. Upon the exercise of this Warrant, the Holder shall
confirm in writing, in a form satisfactory to the Company, that the Shares so
issued are being acquired solely for its own account and not as a nominee for
any other party and not with a view toward resale or distribution thereof in
violation of the Act. This Warrant and the Shares to be issued upon the exercise
hereof (unless registered under the Act and unless, in the case of the Shares,
such Shares may thereupon be sold pursuant to Commission Rule 144(k)) shall be
imprinted with a legend in substantially the following form:

          THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
          SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, TRANSFERRED,
          ASSIGNED

                                       4
<PAGE>

          OR HYPOTHECATED UNLESS THERE IS AN EFFECTIVE REGISTRATION STATEMENT
          UNDER SUCH ACT COVERING SUCH SECURITIES, THE SALE IS 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.

     In addition, this Warrant and the Shares to be issued upon the exercise
hereof shall bear any legends required by the securities laws of any applicable
states.

          (b)  Transferability and Negotiability of Warrant. This Warrant may
               --------------------------------------------
not be transferred or assigned in whole or in part without compliance with all
applicable federal and state securities laws by the transferor and the
transferee (including the delivery of investment representation letters and
legal opinions reasonably satisfactory to the Company, if requested by the
Company). Subject to the provisions of this Warrant with respect to compliance
with the Act, title to this Warrant may be transferred by endorsement and
delivery in the same manner as a negotiable instrument transferable by
endorsement and delivery; provided, however, that prior to the Company's initial
public offering neither this Warrant nor the Shares purchasable with this
Warrant may be transferred to any entity or person which the Company reasonably
determines to be an actual competitor of the Company. The Company shall act
promptly to record transfers of this Warrant on its books, but the Company may
treat the registered holder of this Warrant as the absolute owner of this
Warrant for all purposes, notwithstanding any notice to the contrary.

          (c)  Disposition of Warrant Shares. With respect to any offer, sale,
               -----------------------------
transfer or other disposition of any Shares acquired pursuant to the exercise of
this Warrant prior to registration of such Shares, the Holder and each
subsequent holder of this Warrant agrees to give written notice to the Company
prior thereto, describing briefly the manner thereof, together with a written
opinion of legal counsel for such holder, reasonably satisfactory to the Company
and its legal counsel, if requested by the Company, to the effect that such
offer, sale or other disposition may be effected without registration or
qualification (under the Act or any other federal or state securities laws) of
such Shares and indicating whether or not under the Act, certificates for such
Shares to be sold or otherwise disposed of require any restrictive legend as to
the applicable restrictions on transferability in order to ensure compliance
with the Act. Promptly upon receiving such written notice and reasonably
satisfactory opinion, if so requested, the Company, as promptly as practicable,
shall notify such Holder that such Holder may sell or otherwise dispose of such
Shares, all in accordance with the terms of the notice delivered to the Company.
If a determination has been made pursuant to this subsection (c) that the
opinion of legal counsel for the holder is not reasonably satisfactory to the
Company and its legal counsel, the Company shall so notify the holder promptly
after such determination has been made. Notwithstanding the foregoing, such
Shares may be offered, sold or otherwise disposed of in accordance with Rule
144, provided that the Company shall have been furnished with such information
as the Company may reasonably request to provide a reasonable assurance that the
provisions of Rule 144 have been satisfied. Each certificate representing the
Shares thus transferred (except a transfer pursuant to Rule 144(k) or an
effective registration statement) shall bear a restrictive

                                       5
<PAGE>

legend as to the applicable restrictions on transferability in order to ensure
compliance with the Act, unless in the aforesaid opinion of legal counsel for
the holder, such legend is not required in order to ensure compliance with the
Act. The Company may issue stop transfer instructions to its transfer agent in
connection with such restrictions.

     8.   Rights of Shareholders. No Holder shall be entitled to vote or receive
          ----------------------
dividends or be deemed the holder of Warrant Shares or any other securities of
the Company which may at any time be issuable on the exercise of this Warrant
for any purpose, nor shall anything contained herein be construed to confer upon
the Holder, as such, any of the rights of a shareholder of the Company or any
right to vote for the election of directors or upon any matter submitted to
shareholders at any meeting thereof, or to give or withhold consent to any
corporate action (whether upon any recapitalization, issuance of stock,
reclassification of stock, consolidation, merger, transfer of assets or
otherwise) or to receive notice of meetings, or to receive dividends or
subscription rights or otherwise until this Warrant shall have been exercised
and the Warrant Shares issuable upon exercise hereof shall have become
deliverable, as provided herein.

     9.   Replacement of Warrants. On receipt of evidence reasonably
          -----------------------
satisfactory to the Company of the loss, theft, destruction or mutilation of
this Warrant and, in the case of loss, theft or destruction, on delivery of an
indemnity agreement reasonably satisfactory in form and amount to the Company
or, in the case of mutilation, on surrender and cancellation of this Warrant,
the Company at its expense shall execute and deliver, in lieu of this Warrant, a
new warrant of like tenor.

     10.  Exchange of Warrant. Subject to the other provisions of this Warrant,
          -------------------
on surrender of this Warrant for exchange, properly endorsed and subject to the
provisions of this Warrant with respect to compliance with the Act, the Company
at its expense shall issue to or on the order of the Holder a new warrant or
warrants of like tenor, in the name of the Holder or as the Holder (on payment
by the Holder of any applicable transfer taxes) may direct, for the number of
Shares issuable upon exercise thereof.

     11.  Notices. All notices and other communications from the Company to the
          -------
Holder, or vice versa, shall be deemed delivered and effective when given
personally or three days after being mailed by first-class registered or
certified mail, postage prepaid, at such address as may have been furnished to
the Company or the Holder, as the case may be, in writing by the Company or such
Holder from time to time.

     12.  Waiver. This Warrant and any term hereof may be changed, waived,
          ------
discharged or terminated only by an instrument in writing signed by the party
against which enforcement of such change, waiver, discharge or termination is
sought.

     13.  Governing Law. This Warrant shall be governed by and construed in
          -------------
accordance with the laws of the State of California, as such laws are applied to
agreements entered into in California and to be performed solely by California
residents.

                                       6
<PAGE>

     14.  Titles and Subtitles; Forms of Pronouns. The titles of the Sections
          ---------------------------------------
and Subsections of this Warrant are for convenience only and are not to be
considered in construing this Warrant. All pronouns used in this Warrant shall
be deemed to include masculine, feminine and neuter forms.

Dated: December 3, 1999

                                             NEOPOINT, INC.


                                             By: _______________________________
                                                 William Y. Son, President

                                       7
<PAGE>

                                   EXHIBIT A
                                   ---------

                              NOTICE OF EXERCISE
                  (To be signed only on exercise of Warrant)


TO:  NEOPOINT, INC.

     The undersigned, the holder of the within Warrant, hereby irrevocably
elects to exercise this Warrant for, and to purchase thereunder, shares of
Series C Preferred Stock of NEOPOINT, INC. and herewith makes payment of
$ ________________ therefor in cash and requests that the certificates for such
shares be issued in the name of, and delivered to ______________________________
___________  whose address is __________________________________________________
___________.


Dated:  ________________________             ___________________________________
                                             (Signature must conform to name of
                                             holder as specified on the face of
                                             the Warrant)

                                             ___________________________________

                                             ___________________________________
                                                                       (Address)