[Asterisk-Users] mid-call echo

[Ryan Courtnage]

Hello all,

When it comes to echo cancellation, we have had great success using 
zapata.conf's echo cancellation.  This eliminates the echo at the very 
beginning of phone calls.

However, we now have a small office scenario where the users are hearing 
the introduction of echo several minutes into a phone conversation 
(where the first few minutes of the conversation sound fine):

PSTN <--> TDM400P+FXO (x4) <--> Uniden UIP200 (x4)

Aside from the traffic generated from Asterisk & the SIP clients, there 
is almost zero network activity.

I suspect that the electrical properties of the telco lines changes 
slightly during these phone calls.  Is this a sound theory?

If so, perhaps there are some common telecommunications devices or 
wiring practices that are used in modern high-rise buildings that may be 
causing the problem?  Any advice on how to troubleshoot this?

Thanks in advance
Ryan

ps: I've yet to try defining AGGRESSIVE_SUPPRESSOR in the zaptel source. 
  Is this recommended?