Having a nice conversation with myself currently
Ok - so when I switched off realtime it worked. This also updated the additional_a2billing_sip.conf file of course. So now I switch back to realtime, and then manually change the caller ID value in the config file to something abitrary, do an amportal restart, and lo and behold, the value I just entered comes through. In other words, even though realtime is on, it pulls that value from the flat file.
More bizarre is that the other values, such as the username, are pulled from the database..... I tested that by creating a new user which is not reflected in the config file, but is in the database. I can connect with that user so it has to be taking that value from the database and then seemingly looks for a caller ID in config file when a call is made??