I have some questions:
1) what "number" are you talking about? Their phone number? You will never get an IP address from that
2) If you have their IP, how do you propose to 'find out who it is'?
3) Even if you do find out who it is, then what are you going to do?
Wouldn't it make more sense to get a firewall for Grandma, a good antivirus (or, maybe a Mac), and teach her safe computing?
If the phone has some sort of data-plan on it, subpoena the mobile phone company to reveal all the IP's associated with that phone number.
But, the big question is, what would you do with the IP addresses?
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