asterisk users - Dec 2006 - Error in codec string '=audio 5004 RTP/SAVP 3'

Hello,

I have a problem with a grandstream IP Phone.
The SIP autentication is OK, but when try to call someone I get the message

 --> WARNING[14281] chan_sip.c: Error in codec string '=audio 5004
RTP/SAVP
3'



I tried to change the CODECs (ulaw, alaw, GSM, etc), the result is always
the same.
Tried to change the RTP port but the result is the same.
The grandstream IPhone is behind a NAT, However there are others IP Phones
working fine behind NAT (from other locations, outside my network)
Changed the IP Phone from Grandstream GXP2000 to BT200, got the same probem

The error in the IP PHONE display is "481"
The X-lite Softphone works fine in the same network.


Any help please ?


Regards
Claudemir F. Martins
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