That sounds like chan_sip; you should be using chan_pjsip, especially as chan_sip cannot handle:
cleanly, whereas this can be handled with the match/accept setting for chan_pjsip.
This sounds like a commercial limit, of a type that some people believed no longer exists. At such a low limit there seems no point in trying to implement it, as it is probably easier to let the calls fail.
Having these different may be a problem, although maybe you only need Username for the client URI in the advanced settings. Some more clarification is needed from the provider.
As a general point, obfuscating the provider identity just makes it more difficult to help, and there is enough information that anyone using that provider would recognize them. It looks like it may be yourwhc.co.uk, which seems to be Cloud Direct’s Wholesale Hosted Communications, which appears to be owned by British Telecommunications.