On the 3600 router with Cisco IOS Software Release 12.2(8)T, Cisco bug ID CSCdx63907 causes calls that match the default dial peer to experience this is talker echo, which means that the talker hears an echo of himself. It occurs only on outbound calls and is heard only on the called-party side. Inbound calls do not hear the echo.
As workaround to this issue, use the default dial peer on the inbound Voice over IP (VoIP) leg to the egress gateway.
In order to avoid this, add an additional VoIP dial peer. Issue the incoming called-number command. This command is configured so that outbound calls match this VoIP dial peer, as shown: