<PAGE>

                                                                   Exhibit 10.11
                                 NOTE AGREEMENT

     THIS NOTE AGREEMENT ("Agreement"), dated as of January 1, 2001 is entered
into by and between Corsair Communications, Inc., a Delaware corporation
("Corsair") and Wireless Billing Systems, a California corporation ("Wireless").

                                    RECITALS

     A. Wireless executed that certain Secured Promissory Note dated May 26,
1999, in favor of Corsair in the original principal amount of Two Million Two
Hundred Thirty-Eight Thousand Two Hundred Forty-Two Dollars ($2,238,242.00) (the
"Original Note").

     B. The outstanding principal balance of the Original Note on the date
hereof is One Million Six Hundred Forty-Five Thousand, Eight Hundred Sixty-Six
and 09/100 Dollars ($1,645,866.09).

     C. Wireless's obligations under the Original Note are secured by that
certain Security Agreement dated May 26, 1999 between Wireless, as the debtor
thereunder and Corsair, as the secured party thereunder (the "Security
Agreement"), pursuant to which Corsair has a first priority security interest in
the "Collateral" (as defined in the Security Agreement).

     D. Corsair and Wireless desire to amend and restate the Original Note in
the form of the Amended and Restated Secured Promissory Note attached hereto as
Exhibit A (the "Restated Note").
---------

     E. Corsair and Wireless desire to add to the outstanding principal balance
of the Original Note Fifty Thousand, Five Hundred Twenty-Eight and 09/100
Dollars ($50,528.09), representing additional indebtedness ("Additional
Indebtedness") of Wireless to Corsair not currently evidenced by a promissory
note.

     F. Corsair and Wireless desire to provide Wireless with the option of
obtaining additional financing to which Corsair will subordinate its rights
under the Restated Note and the Security Agreement or extending the term of the
Restated Note, or both.

     NOW THEREFORE, for valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto agree as follows:

     1. Amendment and Restatement of Note. The Original Note shall be amended
        ---------------------------------
and restated in its entirety.

     2. Balance of the Original Note. Corsair and Wireless agree that on the
        ----------------------------
date hereof, the outstanding principal balance of the Original Note (exclusive
of the Additional Indebtedness) is One Million Six Hundred Forty-Five Thousand,
Eight Hundred Sixty-Six and 09/100 Dollars ($1,645,866.09).

<PAGE>


     3. Principal Balance of Note. The original principal balance of the
        -------------------------
Restated Note (including the Additional Indebtedness) shall be One Million Six
Hundred Ninety-Six Thousand Three Hundred Ninety-Four and 18/100 Dollars
($1,696,394.18).

     4. Option to Extend Term. At any time prior to April 30, 2001, Wireless
        ---------------------
shall have the right, in its sole discretion, to extend the term of the Restated
Note from twenty-four (24) months to sixty (60) months from the date of the
Original Note. If Wireless elects to extend the term of the Restated Note, the
interest rate under the Restated Note shall be increased to sixteen percent
(16%) per annum, compounded annually, whether or not Corsair has subordinated
its rights under the Restated Note and Security Agreement as provided in Section
5 of this Agreement.

     5. Subordination of Holder's Right to Payment. If, during the term of the
        ------------------------------------------
Restated Note, Wireless obtains additional financing and the lender requests
that Corsair subordinate its rights to payment under the Restated Note and its
priority in the Collateral to such lender, Corsair shall execute a subordination
agreement in substantially the form attached hereto as Exhibit B or any other
subordination agreement and related documents reasonably requested by such
lender, including any Uniform Commercial Code financing statements requested by
lender to reflect the subordination. If Wireless has not extended the term of
the Restated Note as provided in Section 4 of this Agreement prior to such
subordination, then immediately upon the effectiveness of such subordination,
the interest rate under the Restated Note shall increase from ten percent (10%)
per annum to thirteen percent (13%) per annum, compounded annually. If, however,
Wireless has extended the term of the Restated Note as provided in Section 4 of
this Agreement prior to such subordination, then immediately upon the
effectiveness of such subordination, the interest rate shall be sixteen percent
(16%) per annum, compounded annually, and there shall be no change in the
interest rate as a result of Corsair subordinating its rights under the Restated
Note as provided in this Section.

     6. Re-Calculation of Monthly Payments. Upon any increase in the interest
        ----------------------------------
rate of the Restated Note, the monthly installments of principal and interest
shall be re-calculated to account for the increased amount of interest due with
each payment and Wireless shall continue making equal monthly installments of
principal and interest based on a sixty (60) month amortization schedule at the
revised interest rate, with all unpaid principal and interest accrued thereon
being due upon maturity of the Restated Note.

     7. Further Assurances. The parties agree that at any time, and from time to
        ------------------
time when reasonably requested by the other party, they will execute and deliver
all further instruments, documents and agreements, and at the expense of the
requesting party, take all further action, as the requesting party reasonably
deems necessary to consummate the transactions contemplated by this Agreement.

     8. Captions. Any titles or captions contained in this Agreement are for
        --------
convenience only and shall not be deemed to qualify the meaning of any provision
herein.

     9. Binding Effect. This Agreement shall be binding upon and inure to the
        --------------
benefit of the parties hereto, their heirs, executors, administrators,
successors and assigns.

                                       2

<PAGE>

     10. Entire Agreement. This Agreement and the Restated Note, contain the
         ----------------
entire agreement among the parties hereto with respect to the transactions
contemplated hereby and contains all of the terms and conditions thereof and
supersedes all prior agreements and understandings relating to the subject
matter hereof. No changes or modification of or additions to this Agreement
shall be valid unless the same shall be in writing and signed by all parties
hereto.

     11. Severability. The provisions of this Agreement shall be deemed
         ------------
severable and the invalidity and unenforceability of any one or more of the
provisions hereof shall not affect the validity and enforceability of the other
provisions hereof.

     12. Waiver. The failure of any party hereto to insist, in any one or more
         ------
instances, upon performance of the terms or conditions of this Agreement shall
not be construed as a waiver of future performance of any such term, covenant or
condition and the obligations of each party with respect thereto shall continue
in full force and effect.

     13. Fees and Expenses. The parties hereto shall each pay their own
         -----------------
respective costs, fees and expenses, including, but not limited to, fees and
expenses of counsel, accountants and other professionals, incurred in connection
with the negotiation and execution of this Agreement and the consummation of the
transactions contemplated hereby.

     14. Construction. This Agreement has been reviewed by all parties hereto
         ------------
and their respective attorneys, and all parties have had a full opportunity to
negotiate the contents hereof. The parties expressly waive any common law or
statutory rule of construction that ambiguities be construed against the drafter
of this Agreement.

     15. Governing Law. This Agreement shall be governed by, and construed in
         -------------
accordance with, the laws of the State of California, without regard to conflict
of laws principles.

     16. Counterparts; Facsimile Signatures. This Agreement may be executed in
         ----------------------------------
multiple counterparts, each of which shall be deemed an original without
production of the others and all of which shall constitute one and the same
instrument. A signature delivered by facsimile shall be deemed an original.

                     [The next page is the signature page.]

                                       3

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed as of the day and year first set forth above.

                                         WIRELESS BILLING SYSTEMS
                                         a California corporation


                                         By:  /s/ Joseph R. Simrell
                                            ------------------------------------
                                            Joe Simrell, Assistant Treasurer


                                         CORSAIR COMMUNICATIONS, INC.,
                                         a Delaware corporation


                                         By:  /s/ Martin J. Silver
                                            ------------------------------------

                                         Print Name: Martin J. Silver
                                                    ----------------------------

                                         Title: CFO
                                               ---------------------------------

                                       4

<PAGE>

                                    EXHIBIT A
                                    ---------

              Form of Amended and Restated Secured Promissory Note

                  AMENDED AND RESTATED SECURED PROMISSORY NOTE

$1,696,394.18                                                  January    , 2001
                                                                       ---
                                                              Irvine, California

     FOR VALUE RECEIVED, the undersigned, WIRELESS BILLING SYSTEMS, a California
corporation ("Maker"), promises to pay to CORSAIR COMMUNICATIONS, INC.,
("Holder"), the principal sum of One Million Six Hundred Ninety-Six Thousand,
Three Hundred Ninety-Four and 18/100 Dollars ($1,696,394.18). Interest shall
accrue from the date of this Note on the unpaid principal amount at the rates
set forth in the Note Agreement of even date herewith between Maker and Holder
(the "Note Agreement"). Such principal amount, together with accrued interest,
shall be paid in equal monthly installments based upon a sixty (60) month
amortization schedule (where such sixty (60) month period is the period from May
26, 1999 through May 1, 2004); provided that the amount of the monthly
installments shall be adjusted if the interest rate changes as provided in the
Note Agreement. The unpaid principal amount outstanding under this Note,
together with all accrued interest shall be due and payable in full on the
Maturity Date (as hereinafter defined). If Maker does not extend the maturity of
this Note as provided in Section 4 of the Note Agreement, the Maturity Date
shall be May 1, 2001. If Maker extends the maturity of this Note as provided in
Section 5 of the Note Agreement, the Maturity Date shall be May 1, 2004.

     Upon payment in full of all principal and interest payable under this Note,
this Note will be surrendered to the Maker for cancellation.

     The indebtedness evidenced by this Note is secured pursuant to that certain
Security Agreement dated as of May 26, 1999 between Maker and Holder (the
"Security Agreement").

     The entire unpaid principal sum of this Note, together with accrued and
unpaid interest thereon, shall become immediately due and payable upon any
default by Maker under the Security Agreement, the commission of any act of
bankruptcy by the Maker, the execution by the Maker of a general assignment for
the benefit of creditors, the filing by or against the Maker of a petition in
bankruptcy or any petition for relief under the federal bankruptcy act or the
continuation of such petition without dismissal for a period of thirty (30) days
or more, or the appointment of a receiver or trustee to take possession of the
property or assets of the Maker.

     Payment shall be made in lawful tender of the United States and shall be
credited first to the accrued interest and the remainder applied to principal.
Prepayment of principal, together with accrued interest, may be made at any time
without penalty.

                                       1

<PAGE>

     If action is instituted to collect this Note, the Maker promises to pay all
costs and expenses, including reasonable attorney's fees, incurred in connection
with such action. This Note is not assignable or transferable except to Avery
Communications, Inc. or any wholly-owned subsidiary thereof, and shall be
construed in accordance with the laws of the State of California.

     If at any time any applicable usury law would ever render usurious any
amounts called for under this Note or the other loan documents, then it is
Holder's and Maker's express intention that Maker shall not be required to pay
interest on this Note at a rate in excess of the maximum lawful rate, that the
provisions of this paragraph shall control over all other provisions of this
Note which may be in apparent conflict hereunder, that such excess amount shall
be immediately credited on the principal balance of this Note (or, if this Note
has been fully paid, refunded by Holder to Maker), and the provisions hereof
shall be immediately reformed and the amounts thereafter collectible under this
Note reduced, without the necessity of the execution of any further documents,
so as to comply with the then applicable law, but so as to permit the recovery
of the fullest amount otherwise called for under this Note. Any such crediting
or refund shall not cure or waive any default by Maker under this Note. If at
any time following any reduction in the interest rate payable by Maker there
remains unpaid any principal amount under this Note and the maximum interest
rate allowed by applicable law is increased or eliminated, then the interest
rate payable under this Note shall be readjusted, to the extent not prohibited
by applicable law, so that the total dollar amount of interest payable hereunder
shall be equal to the dollar amount of interest which would have been paid by
Maker without giving effect to the reduction in interest resulting from
compliance with applicable usury laws. The provisions of this clause shall not
apply if, by reason of the retroactive application of any exemption to
California usury law (including but not limited to that available pursuant to
Section 25118 of the California Corporations Code), this Note and the loan
evidenced thereby are exempt from the California usury law.

     This Note amends and restates (and does not constitute an extinguishment or
novation of) that certain Secured Promissory Note dated as of May 26, 1999,
executed and delivered by Maker to Holder in the original principal amount of
Two Million Two Hundred Thirty-Eight Thousand Two Hundred Forty-Two Dollars
($2,238,242.00) (the "Original Note"), a copy of which is attached hereto as
Exhibit A and hereby acknowledged by Maker as a true, correct and complete copy
---------
of the original. This Note amends and restates the Original Note and does not
evidence or effect a refinancing of all or any portion of the indebtedness
evidenced thereby, a release or relinquishment of the priority of the security
interest of Holder in any assets (real and personal) of Maker, including,
without limitation pursuant to the Security Agreement, or a waiver of Maker's
obligation to reimburse Holder for any amounts previously evidenced by the
Original Note.

                                         WIRELESS BILLING SYSTEMS
                                         a California corporation

                                         By:
                                            ------------------------------------
                                                Joe Simrell, Assistant Treasurer

                                       2

<PAGE>

                                    EXHIBIT A
                                    ---------

                              Copy of Original Note

                                       3

<PAGE>

                                    EXHIBIT B
                                    ---------

                         Form of Subordination Agreement

                                       4

<PAGE>

                  AMENDED AND RESTATED SECURED PROMISSORY NOTE

     $1,696,394.18                                               January 1, 2001
                                                              Irvine, California

     FOR VALUE RECEIVED, the undersigned, WIRELESS BILLING SYSTEMS, a California
corporation ("Maker"), promises to pay to CORSAIR COMMUNICATIONS, INC.,
("Holder"), the principal sum of One Million Six Hundred Ninety-Six Thousand,
Three Hundred Ninety-Four and 18/100 Dollars ($1,696,394.18). Interest shall
accrue from the date of this Note on the unpaid principal amount at the rates
set forth in the Note Agreement of even date herewith between Maker and Holder
(the "Note Agreement"). Such principal amount, together with accrued interest,
shall be paid in equal monthly installments based upon a sixty (60) month
amortization schedule (where such sixty (60) month period is the period from May
26, 1999 through May 1, 2004); provided that the amount of the monthly
installments shall be adjusted if the interest rate changes as provided in the
Note Agreement. The unpaid principal amount outstanding under this Note,
together with all accrued interest shall be due and payable in full on the
Maturity Date (as hereinafter defined). If Maker does not extend the maturity of
this Note as provided in Section 4 of the Note Agreement, the Maturity Date
shall be May 1, 2001. If Maker extends the maturity of this Note as provided in
Section 5 of the Note Agreement, the Maturity Date shall be May 1, 2004.

     Upon payment in full of all principal and interest payable under this Note,
this Note will be surrendered to the Maker for cancellation.

     The indebtedness evidenced by this Note is secured pursuant to that certain
Security Agreement dated as of May 26, 1999 between Maker and Holder (the
"Security Agreement").

     The entire unpaid principal sum of this Note, together with accrued and
unpaid interest thereon, shall become immediately due and payable upon any
default by Maker under the Security Agreement, the commission of any act of
bankruptcy by the Maker, the execution by the Maker of a general assignment for
the benefit of creditors, the filing by or against the Maker of a petition in
bankruptcy or any petition for relief under the federal bankruptcy act or the
continuation of such petition without dismissal for a period of thirty (30) days
or more, or the appointment of a receiver or trustee to take possession of the
property or assets of the Maker.

     Payment shall be made in lawful tender of the United States and shall be
credited first to the accrued interest and the remainder applied to principal.
Prepayment of principal, together with accrued interest, may be made at any time
without penalty.

                                       5

<PAGE>

     If action is instituted to collect this Note, the Maker promises to pay all
costs and expenses, including reasonable attorney's fees, incurred in connection
with such action. This Note is not assignable or transferable except to Avery
Communications, Inc. or any wholly-owned subsidiary thereof, and shall be
construed in accordance with the laws of the State of California.

     If at any time any applicable usury law would ever render usurious any
amounts called for under this Note or the other loan documents, then it is
Holder's and Maker's express intention that Maker shall not be required to pay
interest on this Note at a rate in excess of the maximum lawful rate, that the
provisions of this paragraph shall control over all other provisions of this
Note which may be in apparent conflict hereunder, that such excess amount shall
be immediately credited on the principal balance of this Note (or, if this Note
has been fully paid, refunded by Holder to Maker), and the provisions hereof
shall be immediately reformed and the amounts thereafter collectible under this
Note reduced, without the necessity of the execution of any further documents,
so as to comply with the then applicable law, but so as to permit the recovery
of the fullest amount otherwise called for under this Note. Any such crediting
or refund shall not cure or waive any default by Maker under this Note. If at
any time following any reduction in the interest rate payable by Maker there
remains unpaid any principal amount under this Note and the maximum interest
rate allowed by applicable law is increased or eliminated, then the interest
rate payable under this Note shall be readjusted, to the extent not prohibited
by applicable law, so that the total dollar amount of interest payable hereunder
shall be equal to the dollar amount of interest which would have been paid by
Maker without giving effect to the reduction in interest resulting from
compliance with applicable usury laws. The provisions of this clause shall not
apply if, by reason of the retroactive application of any exemption to
California usury law (including but not limited to that available pursuant to
Section 25118 of the California Corporations Code), this Note and the loan
evidenced thereby are exempt from the California usury law.

     This Note amends and restates (and does not constitute an extinguishment or
novation of) that certain Secured Promissory Note dated as of May 26, 1999,
executed and delivered by Maker to Holder in the original principal amount of
Two Million Two Hundred Thirty-Eight Thousand Two Hundred Forty-Two Dollars
($2,238,242.00) (the "Original Note"), a copy of which is attached hereto as
Exhibit A and hereby acknowledged by Maker as a true, correct and complete copy
---------
of the original. This Note amends and restates the Original Note and does not
evidence or effect a refinancing of all or any portion of the indebtedness
evidenced thereby, a release or relinquishment of the priority of the security
interest of Holder in any assets (real and personal) of Maker, including,
without limitation pursuant to the Security Agreement, or a waiver of Maker's
obligation to reimburse Holder for any amounts previously evidenced by the
Original Note.

                                         WIRELESS BILLING SYSTEMS
                                         a California corporation


                                         By:  /s/ Joseph R. Simrell
                                            -----------------------------------
                                            Joe Simrell, Assistant Treasurer

                                       6

<PAGE>

                                    EXHIBIT A
                                    ---------

                              Copy of Original Note

                                       7