I appreciate this has probably been asked time and time before, but I can not find any documentation on it which works in my situation. I have tried nathelper.cfg, rtp.cfg, al.cfg but to no avail. I have even tried home grown cfg's |---------sipsak for external registration | | rtpproxy \|/ + sip server <-> ser <-> asterisk ^ ^ | | \|/ \|/ sip phone mgcp/iax phone The ser and asterisk programs are running on the same box. This box has an external and internal interface. ser listens to 5060, asterisk sip listens to 5061. Internal phones connect to the internal interface of this box. I used fix_nated_contact to modify the contact header, however, looking at the xlog output this does not appear to be happenning. /usr/local/sbin/ser[7077]: time [Tue May 3 17:04:33 2005] method <REGISTER> r-uri <iptel.tgfslp.dalmany.co.uk> src_ip <192.168.4.5> contact header <Line A <sip:5561@192.168.4.5:5061>;expires=3600> Does anyone have any suggestions on how to fix this problem? Unfortunately, I need this setup to proxy rtp through the box. Many thanks, Spencer