I think this must be coincidence; only 2 SIMs (I’m assuming yours and your friend’s) have had calls from a number starting 07511. At this point, I’m afraid there’s nothing we can do on our side.
Kind of. Not at the network level, but we could use client-side libraries for iOS and Android to block numbers. Given your permission, we could block every number on a list of known spam numbers (although this hardly solves the problem – there’s the danger of blocking legitimate numbers, and we’d need to keep it updated), or add a button on the call log to “Block this number,” but that would be after the fact…
It’s also possible to show more info on the calling screen, so even if we didn’t block the number, we could show “Likely spam number” or something.
Again, this is only client-side, and wouldn’t work on all handsets.