Default [Q] IMEI shows 0/01, is my i9505 rooted?

Hello everybody!

I have a Galaxy S4 (GT-i9505) which shows "No network service", even though I almost always can access internet. However I can't make calls or send text messages. I just realized it shows IMEI 0/01 when i dial *#06#. The device status says official, but I can't update the phone (currently on android 4.2.2). When I try to do it trough Kies it says that the firmware isn't supported. Does this mean the previous owner has flashed the phone? The problem with calling and texting only seems to appear with some carriers.

I don't want to void the warranty if it isn't already and I have no experience of flashing phones at all, so I'd prefer to fix it without doing that! Does anybody know if it's possible? The phone is unlocked and is first bought from Telenor in Sweden (in case that makes any difference).