I've found the current retail page confirming that they'll sell one of Sim to a new owner.

So I'll investigate that one first.

If Simwood would still give you one,

if the factory makes the entire Twilio vehicle work around when necessary,

you could retain it.

Twilio 113 number,

sit your 3TBX and have 3TBX extension 201,

Simwood Sim, iPhone incoming.

Caller, draw 114, Twilio 3TBX extension 201,

iPhone Simwood Sim.

3TBX, Twilio PSTN.

Your 3TBX therefore remains your PBX.

Twilio remains your numbers trunk provider.

And the iPhone effectively becomes a real cellular PBX handset.

That is substantially more elegant than DZA,

if Simwood's current service still permits it.

And your requirements are sufficiently unusual that I'd specifically ask Simwood.

Open quote.

Can you currently supply a UK mobile Sim which can register as a SIP endpoint to my own...