Just a correction for the above and a minor update - obviously the SIP registration accounts I created are in regfile.conf, not accfile.conf as I erroneously mentioned in the message above.
However, reading around the forum it looks like the problem might be related to the way the IPs are written in the SIP messages/headers by the remote SIP client (the OBI100 ATA device). Since Yate cannot act like a SIP proxy it cannot rewrite these IPs, so I suppose they are expected to come prepared correctly by the ATA device, or perhaps by a SIP proxy? Also, I know I have the option to configure the use of a STUN server in these ATA devices - but I tried that and it didn't seem to help..
Interestingly enough, a Yate client as a remote SIP client seems to allow SIP registration to my Yate SIP server, so it's clearly something weird in the SIP messages coming from these obi100 devices when they are outside the LAN.
So my next question would be: is there any way to force Yate to modify/correct these SIP headers (perhaps in the response?) and somehow be able to continue the registration process for the remote device? The frustrating part is that it seems to me that all the needed info (local IP, public IP) is there in those headers when the request comes from the SIP device..
If Yate cannot do that, perhaps I should install a minimal SIP proxy (something like siproxd) to try to get around this problem? Any advice would be greatly appreciated..