Called party sometimes can't hear calling agent

We are running an Asterisk setup in a call center.
The Agents use SIP-softphones to connect to the Asterisk-server via local office LAN (no NAT). Asterisk is connected to an E1 line with a Digium Te122. The Agents are using Ekiga for Windows as softphone ATM.

Our problem: sometimes, after being a couple of minutes into a call, the called party can’t hear our agent anymore, the agent can hear the called party fine. this happens randomly in every 20th call or so, it doesn’t happen at all for some agents.

i am puzzled where to start to search for the error? is this probably an Asterisk/zaptel problem or would the error rather be on the softphone/windows side? The microphones are still working when that happens, the talk-volume indicator bar is working in ekiga, but the called party can’t hear.

tnx for any ideas
greetings wonder

the echo canceler in ekiga is buggy. after turning it of on the clients, everything worked as expected…