Just to clarify the request, are you looking for a setting on the trunk configuration, which customizes the realm for registration purposes to something other than pbxes.org?
No, not really. I just want to register with a SIP provider using an account that requires a realm/domain different from the provider's SIP server's FQDN.
On the other hand, it would be nice to be able to receive SIP calls via a custom domain, expecially for resellers. Does PBXes have this facility, i.e. support _sip._udp SRV DNS records?