In account settings, distinguish between "Display name", Caller ID, "Caller ID name" / CNAM (and make it clear for the user)
In SIP (and arguably in the POTS / PSTN) there's the (nick)name of the account in the Calls accounts list (and wherever it may be used in the UI, such as the incoming call notifications/status screens, perhaps) vs the Caller ID name (we often commonly say "Caller ID", but that's the phone number part, ye olde thing from the fifties... "Caller ID name" is a separate extension on top of it, as far as I understand... confusing stuff).
So basically the UI would need to expose:
- "Display name (for the Calls UI)"
- "Caller ID number"
- "Caller ID name (CNAM)"
- Account username (for logging in)
Maybe other things? I'm not an expert on this, but see these further readings from my VoIP provider: