That could really work, especially as most frauds like this are performed using PAYG mobile SIMS, however, they're more likely to exploit it with a 'number unkn

That could really work, especially as most frauds like this are performed using PAYG mobile SIMS, however, they're more likely to exploit it with a 'number unknown' similar to what we have today with Caller ID. This could work really well, and be easy to implement, with a VoIP service too.