Hi. Codecs are negotiated by both sides of the rtp/sdp session, that may explain the silences in sip-to-sip sessions. However, I dont know how to select codecs in the N9's SIP client... OTOH, I have an android tablet with CSipSimple voip client and it 'talks' ok with N9 client, or with regular POTS telephone lines, this may help you to find out wether the problem is local or remote. I use a small PBX at home with Asterisk. Regards.