wait_for_silence not working through external gateway

I'm attempting to detect when there is silence on the call in order to play audio file. If I dial to an internal extension the wait_for_silence and detect_silence commands are working fine. However if dial out through an external gateway to PSTN I don't get expected results. Silence is always detected, even if there is voice speaking. I can detect DTMF commands fine... Is there any known issue or parameter required on the gateway in order to detect the audio / silence. Thanks in advance! -- Sent from: http://freeswitch-users.2379917.n2.nabble.com/