I have the same issue. It's just blank under Settings->Cellular->Active Network. I don't think it's a big deal as long as your text and calls work, unless you want the phone tile to display it. I'm on verizon with an HTC 8x.
Oh please don't promote the HTC 8x its a ******y phone anyway I found the solution to my problem, my operator took to long to active the sim and 1 hour back I restarted my phone and I founded a signal so its not windows phones fault or nokia's