asterisk users - May 2008 - sound quality drop after call transfer

Hi,

  I noticed that the sound quality and volume drop rapidly after call
transfer.  Any clue can tune it?
Below is my setting.

Phone A --calls--> Phone B --blind transfer--> Phone C

A and B talk without problem.  After blind transfer, the volume and
sound quality drop rapidly in the talk between A & C.
asterisk version:
asterisk 1.4.19.1
asterisk-addon 1.4.6
zaptel 1.4.10