Galaxy s4 question

[asphyxia]

A friend of mine came today with his galaxy s4 because it has a problem.

He bought a galaxy s4 from :quote: "a friend of my wife who wanted to change his phone" 2 months ago and now it started having a huge problem.
Basically when he turns on the phone it works for a few minutes and then the network signal dissappears, he cannot be called and he cannot call no matter what he does. All the rest works fine, no problem at all but to be able to make a call he has to restart the phone.

What I tried is this:
-changed his sim with mine and the problem is still there (different carriers)
-updated the firmware to the latest no-brand sgs4 firmware (was branded before) but the problem is still there.

Now I want to try rooting his phone and install a custom rom from xda but an idea came in my mind. Since he has no bill and no warranty I think he bought a phone which was under contract. The guy who made the contract stopped paying it after he sold the phone and now that 2 months have passed the carrier could have decided to lock the imei of the phone.
Is there any way to know if the imei is locked? I doubt there is an unlock process if you don't have the bill and go to who sold it but I would like to know what to tell him tomorrow