Sip.conf register line

I’m having problems with the register=> line in sip.conf…

If I use a full domain name in register=> it works fine, but when I use a section name (defined in sip.conf) instead of the host it fails with this;

[Jul 22 17:58:21] WARNING[18156]: chan_sip.c:7324 transmit_register: Probably a DNS error for registration to XXXXXXXX@orbtalk, trying REGISTER again (after 20 seconds)

But if the register line is

register=>XXXXXXX:YYYYYYYY@talk.orbtalk.co.uk

It works fine…

I need to use the section name instead of a FQDN in this case to define the incoming context for calls from this sip provider.

Any suggestions