IMEI null, can't make any calls topic

So, when I type *#06#, the only thing that I get is IMEI null and I can't make any calls. Tried many times to switch the operating system from 4.2/4.3 to 5.0 but doesn't work, please, don't ask questions about it, because the phone isn't mine, that means I don't have acces to it whenever I want, so please, any solutions ? :)