Are you allowed to see your significant other's cell phone?

My SO used to always let me see his phone in the beginning of our relationship. Now everytime I ask he says no and quickly changes the subject. I've tried to sneak it a couple times but it never seems to work out because he always has it on him & won't bring it out in front of me. There's been times where he has hidden it too when i'm over at his place! Is this the case with many others too?