do you mean user's personal phone number? I'm _not_ questioning your security here, but what if some hacker _somehow_ managed to intrude SF database and collected users phone numbers? (users who browsed SF with cellphone.)
i had a mobile forum and there was a really annoying person but if i banned his ip it would have banned mine since we were on the same network.
in the end i made a script that checked IP and phone (including firmware) and if it matched his - no access i might've banned some other people but unfortunatly thats the most specific way of banning through mobile networks - nothing that the phone sends you is unique.