You can do anything really. Some people have tested hacking a car with a mobile phone for instance.
For the other way around, these days a phone is really just a micro computer. It has its own OS, its own code (Iphones) and its own security. My favorite way to hack mobile phones is through blue tooth these days, and just download data that way. Now if you're really clever some people have been making back doors into computers with phones and vice versa. From there you can record, listen in, control, all sorts of things.
#2: You could call them...
This sounds a bit like the "What can i do if I get some one's IP address" question one may see on a lot of forums, and you're going to get similar results. You can find their location...to an exstent, you could contact them, really it depends on what you wanna do.
Something that may interest you is number spoofing. Making a call from a different number than what you're calling from. Say, their service provider, requiring a password....
3# no.
4# Just start looking for what you want...
It really depends on your end goal.
