I am currently on my fiancé’s phone plan, but I pay for my phone and bill. If we argue or break up, can he immediately take the phone back or cut off my line? Would the same apply if we are out of town and he takes my phone then leaves me there? All of my information is in the phone and I am signed into all of my personal accounts, does that make a difference?

It is his plan. He can do as he chooses with that plan. Basically he is the legal owner of that phone. If he wants to take the phone he can. If he wants to cut off your line, he can.

He can dump you for any reason whenever he wants. That's goes for your phone too. You would have to pay off the phone or get your own account. Call the plan provider to get the details of how to cover your own a$$.

Agreed, yes it is his phone, his plan. Your only right may be to sue him for the money you paid on the phone and the plan later after he takes.

Of course how is he going to "take it" unless you give it to him. He can not physically take it from you. If you refuse to give it to him, he can sue you for the value.

On a side note. If you don't trust him, not to take the phone and leave you on the side of the road on a trip. Time to get a new phone plan of your own and dump the boyfriend first