Mark's wife got a mobile phone through Verizon and he had it unlocked to use with an MVNO carrier, but it doesn't work. Leo says that it could be due to radio frequency issues. Leo says that unlocking doesn't guarantee that he'll be able to use the phone anywhere. But it's the first step. That's why everyone is going to LTE. That will smooth out the system and make it easier to transfer from one carrier to another.
Mark also can't access his voicemail because it thinks he's still on Verizon. Leo says that's a porting issue and he'll need to contact his new carrier and tell them he's moved from Verizon, and he'll need the proper settings for voicemail and data. He could also try rooting his phone. Mark should go to XDA Developers and look up the instructions for his EXACT model of phone. Then he should follow the instructions to the letter. This could not only help solve the issue, but it will open up a host of new options to him.