Noise when transferring inbound ZAP call to outbound

Any suggestions are appreciated. (crossposted to trixbox forum)

We have 6 inbound/outbound POTS lines on 2 TDM400 cards. The system works very will for both inbound and outbound calls. However, if an inbound call is transferred to an outbound line, noise (much like modem squeal) is heard.

For example:

  1. A call is answered - inbound from PSTN
  2. Receptionist answers and realizes that the call should be transferred to the recipient’s cell phone.
  3. The receptionist calls the cell to announce.
  4. The oubound call is answered and the announcement is made.
  5. The transfer is completed.
  6. Both ends cut from good sound to modem-like squeal as soon as the transfer is made.

System: Dual TDM400
Phones: Snom 320
Interesting note: inbound calls are recieved on line1 up to line6
Outbound calls are made down from line6 to line1 so
The inbound/outbound leg of the call are likely on different TDM400 cards.