[Serusers] Problem with ACK

[Amozurrutia Jesus]

I'm implementing the scenario shown below.

   ___      ______
  / 0 \ ---/      \   |
   /_\     | ATA1 |---|    ____     _  _  _  _     _____ 
           \______/   |   /    \   / \/ \/ \/ \   /     \
                      |---| FW |---|  IP Net  |---| CCM |
                      |   \____/   \_/\_/\_/\_/   \_____/
                      |             |        |           
                      |             |        |           
                                    |        |           
                                   ---      ---          
                                  /   \    /   \         
                                  |NAT|    |SER|         
                                  |-T |    |   |
                                  \___/    \___/         

When calling between the CCM (Cisco CallManager) and the ATA, SER simply
ignores the call ACK.
The ACK looks like:

  ACK sip:5559853979*sip1.mcm.net.mx=X.X.71.2+17081 at X.X.81.92:5063 SIP/2.0
  Via: SIP/2.0/UDP  X.X.67.218:5060;branch=z9hG4bK29b2c750
  From: "5559852600" <sip:5559852600 at X.X.67.218>;tag=16781758
  To: <sip:5559853979 at mcm.net.mx>;tag=2602576443
  Date: Tue, 30 Nov 2004 23:53:15 GMT
  Call-ID: fef8ed00-1da1612d-24d-da4334c8 at X.X.67.218
  Route: <sip:5559853979 at X.X.81.94;ftag=16781758;lr>
  Max-Forwards: 70
  CSeq: 101 ACK
  Content-Length: 0

The call flow:

     CallManager         SIP Server             ATA
          |                   |                   |
          |-- INVITE -------->|                   |
          |<-- trying --------|                   |
          |                   |-- INVITE -------->| 
          |                   |<-- Trying --------| 
          |                   |<-- Ringing -------| 
          |<-- Ringing -------|                   |
          |                   |<-- OK ------------| 
          |<-- OK ------------|                   |
          |-- ACK ----------->|                   |
          |                   |<-- OK ------------| 
          |<-- OK ------------|                   |
          |-- ACK ----------->|                   |
          |                   |<-- OK ------------|
          |<-- OK ------------|                   |
          |-- ACK ----------->|                   |
          |                   |<-- OK ------------| 
          |<-- OK ------------|                   |
          |-- ACK ----------->|                   |
          |                   |<-- OK ------------| 
                ......

My guess is that SER does not accept the URI
"5559853979*sip1.mcm.net.mx=X.X.71.2+17081 at X.X.81.92:5063" because it
contains "*+=" signs ore something similar.
When calling from ATA - ATA there is no preoblem because ATAs construct the
ACK message different (URI and Rote flipped).

Any ideas? 

Thanks in advance,

Jesus