Wow that is so illegal. Yeah, just give the phone # to the police, and if there is no number, tell the police about it anyway, and they will find out.
Not quite in the UK. You need to register your phone number with the 'caller preference service' to make un-solicited calls illegal - problem is they only let you register land lines.
At best the offender will get their phone blocked, but it wont stop them getting another.
Yeah, the UK sucks for this stuff.