This post is in case someone else has this problem.
The cause of the issue turned out to be one of the site technicians
having the same extension registering from his laptop as the ATA we
were testing. His laptop wasn’t always connected to the voice network
and the soft-phone wasn’t always on. Sometimes we were able to make
calls from the ATA we were testing and sometimes we would get the
problem described below.
Everything came to light when his soft-phone registered (throwing up a
non-voice network IP address) whilst I was watching the CLI and we
realised the problem…
Hope this helps someone.