The much more interesting option, an actual PBX SIM.

I found one provider that is very specifically what you've been describing, SIMWOOD.

Historically, SIMWOOD Mobile explicitly allowed its SIMs to be configured as SIP endpoints or registered to your own PBX.

Their documentation describes configuring a mobile SIM to register to your PBX.

That means the architecture can effectively feed your free PBX SIP SIMWOOD mobile network cellular SIM in your iPhone.

In other words, the iPhone isn't running GroundWire and isn't doing SIP.

The SIM mobile network is the PBX endpoint.

That's exactly the distinction you've been looking for.

There is one significant qualification.

The explicit page I found describing SIM PBX registration is old and says it was available to their mobile development customers.

SIMWOOD is still very much operating in 2026 and still talks about convergence between mobile and PBX.

But I haven't yet.