Yes, if iOS had a per-line dial prefix rule, your dual-SIM idea would be extremely neat.

SIM 1, ordinary personal number data, dial contacts.

SIM 2, selecting this line will transform 077013456 into something like TwilioVisa.077012356-TAC.

Twilio answers, receives the DTMF and places the call showing your 0114 business number.

Unfortunately, I can't find a native iPhone setting that says, when using this SIM, prepend these digits to every call.

Apple supports pauses, equals two seconds, and waits within a stored telephone number, but not a per-SIM dialing prefix.

A shortcut might get us close, but it wouldn't make the ordinary phone app magically rewrite every number based on which SIM you selected.

Your extra contact number approach therefore remains the most native-looking solution,

unless we find it useful.

The carrier-slash-SIM that does the PBX integration itself.