I’m trying to avoid having different feature codes for this and any sort of pre-configuration. I am ideally wanting to dial the same feature code, with a different ending - this being the phone number that I want to set as the caller ID.
Administrations are clamping down on spoofed caller IDs. +44 is rather behind +1 on this, claiming they need to wait until 2025, when POTS is turned off, but the US position is that a service provider must classify caller IDs as one of, the real caller ID, not the real caller ID, but the true origin is on record, or true origin unknown, and that anyone who provides telephony services to others must register, with the authorities, as a service providre,and must certify numbers in this way.
If the system works, you can expect anyone receiving one of your customised caller ID to not answer the phone, on the basis that the call appears to be a scam.
It may be possible to get a small set of numbers fully trusted, but that would better be handled by sending the index number within the trusted list.