Someone cannot see your facebook profile "on your IP address" .. they can very easily and simply just go to Facebook.com to see it. If your profile is private, having your IP address will not help them in any way.
Nobody can see your messages unless they have access to the Facebook system/servers or have your Facebook password.
If someone has your IP address about all they might be able to see is the type of device you are using.
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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