Exhibit 10.20  Mr. Guru
Pai 7
Technology Park Dr. Westford,
MA 01886  February 18, 2010  Dear
Guru:  This
letter confirms that, effective February 8, 2010 (the Commencement Date),
you have assumed the role of Executive Vice President and Chief Operating Officer
(COO) of Sonus, Inc. (the Company). 
Your duties as COO will be as determined by the Chief Executive Officer
from time to time.  In this position, you
will receive:  1.             Base Salary.          An increase in your base salary from
$275,000 to $375,000, effective as of February 8, 2010.  2.             Target Bonus.       Subject to the applicable plan or program
in effect at the time, a bonus at target equal to seventy percent (70%) of your
then current base salary.  3.             Stock Option Grants.  You will be granted non-qualified options to
purchase 365,000 shares of common stock of the Company, $.001 par value per
share (the Common Stock), under the Companys 2007 Stock Incentive Plan, as
Amended (the Plan), subject to the terms of the Plan and the terms of a stock
option agreement to be entered into between you and the Company, which shall
reflect the terms of this letter.  The
grant date will be on the earliest 15thday of the
month that next follows the Commencement Date or the first business day
thereafter if that day is not a business day or is a bank holiday.  The per share exercise price will be the per
share closing price of the Common Stock on such grant date.  Subject to the provisions of this letter, the
option shall vest and become exercisable as follows: (i) 25% of the shares
of Common Stock (91,250 shares) shall vest on the first anniversary of the
Commencement Date and (ii) the remaining 75% of the shares of Common Stock
(273,750 shares) shall vest in equal monthly increments thereafter through the
fourth anniversary of the Commencement Date.  4.             Restricted Stock Grants;
Performance Stock Grants.  You will
be granted restricted shares of Common Stock as follows:  (a)           You will be granted 175,000
restricted shares of Common Stock (the Restricted Shares) under the Plan,
subject to the terms of the Plan and a restricted stock agreement to be entered
into between you and the Company, which shall reflect the terms of this
Agreement (the Restricted Stock Agreement). 
The grant date will be on the earliest 15thday of a
month that next follows the Commencement Date or the first business day
thereafter if that day is not a business day or is a bank holiday (the Restricted
Shares Grant Date).  The Restricted
Shares shall vest as follows: (i) 25% of the Restricted Shares (43,750
shares) shall vest on the first anniversary of the Commencement Date and (ii) 75%
of the Restricted Shares (131,250 shares) shall vest in


six equal increments semi-annually thereafter
through the fourth anniversary of the Commencement Date.  (b)           In addition to the grant of
Restricted Shares, you will also be eligible to receive 175,000 restricted
shares of Common Stock (the Performance Shares), subject to the terms of the
Plan and the Restricted Stock Agreement. 
The grant of Performance Shares shall be conditioned upon your
achievement and/or the Companys achievement (as determined by the Chief
Executive Officer) of certain performance metrics during your employment with
the Company.  The grant date of
Performance Shares, if applicable, will be on the earliest 15thday of a month that next follows the month
that the performance metrics are determined to have been met, if at all, or the
first business day thereafter if that day is not a business day or is a bank
holiday.  The Performance Shares shall
vest in full on the date of such grant.  You
are, and will remain, an employee at will; nothing in this letter constitutes a
guaranty of employment for any particular period.  Except as modified by this letter, the terms
of your employment letter dated December 11, 2008 shall remain in full
force and effect.   Sincerely,/s/
  Kathy HarrisKathy
  HarrisVP
  of Human ResourcesACCEPTED:/s/ Guru PaiGuru
  PaiDate:
  2/18/10