I have the same problem. Had to set Connection type to 2G/3G, in order to make a call. I can send/receive SMS but no MMS. The only strange thing is that my Nexus 5 (hammerhead) works. Although, my connection type is set to '2G/3G', phone's toolbar shows H or L icon and I can make calls, receive, and send SMS/MMS. My carrier is Red Pocket GSMA (AT&T)