Sip no audio


i frequently experience this problem after upgrading to asterisk-1.2.16, zaptel-1.2.15. when i make/receive call from SIP to SIP the one party hear but the other not
and i receive this error in cli

one sip client is behind nat. and either sip cleint with no nat has the audio problem or the sip client behind nat(nat=yes for sip client behind)