[TMSZ]

I do believe that Cisco SIP implementation in 7000 series phones is non-interoperable by design. One major issue is that they are sending SIP messages from different port than the port they are receiving on. Because of this they practically do not work with any kind of NAT and I haven't heard of any consumer service provider that would work with this (the standard behavior is to assume that port passed in Contact line might be incorrect). Some of phones from that line have "NAT" true/false setting, but some don't - when tracing changelog I saw that this setting was removed in later firmware version, but for many phones this earlier firmware version was never available.