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                                                                 Exhibit 10.4

                                ACME PACKET, INC.

                        INCENTIVE STOCK OPTION AGREEMENT

     This INCENTIVE STOCK OPTION AGREEMENT, dated as of December 23, 2005 (this
"AGREEMENT"), is between ACME PACKET, INC., a Delaware corporation (the
"COMPANY"), and Pat MeLampy (the "OPTIONEE"). Capitalized terms used herein
without definition shall have the meaning ascribed to such terms in the
Company's 2000 Equity Incentive Plan, a copy of which is attached hereto as
EXHIBIT A (the "PLAN").

     1.   GRANT OF OPTION. Pursuant to the Plan, the Company grants to the
Optionee an option (the "OPTION") to purchase from the Company all or any number
of an aggregate of 300,000 shares, subject to adjustment pursuant to Section 8
of the Plan (the "OPTION SHARES"), of the Company's common stock, $.001 par
value per share, at a price of $1.10 per share. The Option is granted as of
December 23, 2005(the "GRANT DATE").

     2.   CHARACTER OF OPTION. The Option is intended to be treated as an
"incentive stock option" within the meaning of Section 422 of the Internal
Revenue Code of 1986, as amended (the "CODE").

     3.   DURATION OF OPTION. Unless subject to earlier expiration or
termination pursuant to the terms of the Plan, the Option shall expire on five
year anniversary of the Grant Date.

     4.   EXERCISE OF OPTION.

     (a)  Vesting Schedule. Until its expiration or termination, the Option may
be exercised, in the manner specified in Section 7.1(g) of the Plan, for 25% of
the Option Shares on December 23, 2006, and for the balance of the Option Shares
in monthly installments, with each installment being as nearly equal as
practicable (as determined by the Company in its reasonable discretion), at the
end of each calendar month beginning January 31, 2007 and ending December 31,
2009. The provisions of this Section 4(a) shall be subject to the provisions of
Section 7.1(e) of the Plan.

     (b)  ACCELERATION OF VESTING. Notwithstanding anything in Section 4(a)
above to the contrary but subject to the provisions of Section 4(c) below, in
the event that (A) a Sale of the Company Transaction (as defined in Section 4(d)
below) occurs prior to the time that the Option is not exercisable in full for
all of the Option Shares, and (B) (i) the Optionee suffers any material adverse
change in authorities, duties or responsibilities, (ii) (ii) the Optionee
voluntarily terminates his employment with the Company following any relocation
of the Optionee (without his written consent) by the Company to a location that
increases Optionee's commute prior to such relocation by more than fifty (50)
miles or (iii) the Company terminates the Optionee's employment with the Company
for any reason or no reason (other than Cause, as such term is defined in
Section 4(d) below) in either case within 365 days of a Sale of the Company
Transaction, then the exercisability

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of the Option shall be accelerated such that the Option shall become exercisable
for an additional number of Option Shares equal to fifty percent (50%) of the
then Unvested Option Shares (as defined below in Section 4(d) below). The
foregoing provisions of this Section 4(b) shall be implemented ratably across
all Unvested Option Shares that are subject to the Option immediately prior to
such Sale of the Company Transaction regardless of when the Option would have
otherwise become exercisable for such Unvested Option Shares pursuant to
Section 4(a) above. Notwithstanding anything express or implied to the contrary
in the foregoing provisions of Section 4(a) and this Section 4(b), the Option
may, as provided in Section 7.1(d) of the Plan, at any time be further
accelerated at the discretion of the Committee, PROVIDED that, without the
consent of the Optionee, such acceleration would not cause the Option to fail to
comply with the provisions of Section 422 of the Code.

     (c)  SALE OF THE COMPANY. Following a Sale of the Company Transaction,
employment of the Optionee by any person or entity that is the successor or
acquiror (each a "SUCCESSOR ENTITY") of the Company shall be treated in the same
manner under this Agreement as if the Optionee were employed by the Company, and
in such context any reference in this Agreement to the Company shall be deemed
to be a reference to the Successor Entity to the Company

     (d)  DEFINITIONS.

          "CAUSE" shall mean (i) if the Optionee is convicted of, or pleads
          guilty or no contest to, a felony or any crime involving moral
          turpitude, deceit, dishonesty or fraud; (ii) any act of embezzlement,
          theft, sexual harassment, discrimination, fraud or other acts of a
          criminal nature by the Optionee in his dealings with the Company or
          its employees or representatives, as determined by the Board of
          Directors of the Company; (iii) the breach by the Optionee of any
          material term of an agreement with the Company or any of its
          subsidiaries, including covenants not to compete and provisions
          relating to confidential information and intellectual property rights;
          or (iv) any failure by the Optionee to comply with a specific
          directive given by the Company's executive officers or Board of
          Directors which failure has not been cured within 30 days after
          written notice from the Company.

          "SALE OF THE COMPANY TRANSACTION" shall mean any Transaction in which
          the shareholders of the Company immediately prior to such Transaction
          do not own or hold, immediately after consummation of such
          Transaction, shares of capital stock of the surviving person or entity
          or acquiring person or entity, as applicable, in connection with such
          Transaction representing at least a majority of the total voting power
          of the outstanding capital stock of such surviving person or entity or
          such acquiring person or entity, as the case may be.

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          "TRANSACTION" shall mean shall mean any merger or consolidation of the
          Company with or into another person or entity, the sale or transfer of
          all or substantially all of the assets of the Company, or the sale or
          transfer by the stockholders of the Company of all outstanding shares
          of capital stock of the Company, in each case in a single transaction
          or in a series of related transactions.

          "UNVESTED OPTION SHARES" shall mean, at the relevant time of reference
          thereto, those Option Shares for which the Option has not yet become
          exercisable at such time pursuant to Section 4(a) and without giving
          effect to the provisions of Section 4(b) above.

     5.   TERMINATION OF ASSOCIATION WITH THE COMPANY. If the Optionee's
employment with the Company is terminated, whether voluntarily or otherwise, the
Option, to the extent the Option is exercisable on the date of termination, may
be exercised by the Optionee, but only within 90 days after the Optionee ceases
to be an employee of the Company, unless terminated earlier by its terms. For
purposes of this Section 5, military or sick leave shall not be deemed a
termination of employment, PROVIDED that it does not exceed the longer of 120
days or the period during which the absent Optionee's reemployment rights, if
any, are guaranteed by statute or by contract. Death or disability (other than
sick leave as specified in the preceding sentence) shall be deemed to be a
termination of employment.

     6.   TRANSFER OF OPTION. Other than as expressly permitted by the
provisions of Section 7.1(f) of the Plan, the Option may not be transferred
except by will or the laws of descent and distribution and, during the lifetime
of the Optionee, may be exercised only by the Optionee.

     7.   INCORPORATION OF PLAN TERMS. The Option is granted subject to all of
the applicable terms and provisions of the Plan, including, but not limited to,
the limitations on the Company's obligation to deliver Optioned Shares upon
exercise set forth in Section 9.1 (Violation of Law), Section 9.2 (Corporate
Restrictions on Rights in Stock), Section 9.3 (Investment Representations) and
Section 9.7 (Tax Withholding).

     8.   MISCELLANEOUS. This Agreement shall be construed and enforced in
accordance with the internal, substantive laws of The Commonwealth of
Massachusetts and shall be binding upon and inure to the benefit of any
successor or assign of the Company and any executor, administrator, trustee,
guardian, or other legal representative of the Optionee.


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     IN WITNESS WHEREOF, the parties have executed this Incentive Stock Option
Agreement as a sealed instrument as of the date first above written.

ACME PACKET, INC.                         OPTIONEE


By: /s/ Andrew D. Ory                     /s/ Patrick MeLampy
    -------------------------------       ---------------------------
    Name:
    Title:

                                              Optionee's Address:

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                                                                       EXHIBIT A

                           2000 EQUITY INCENTIVE PLAN