My wife and I are suffering the same issue with the Lumia 521. They are supposedly working on the issue now, and support will get back to us in another couple of days. The thing is is that I got the Lumia 21 first, and it was working so well after three weeks, that I decided to get my wife one as well. After she had it about a week, then the problems began for both of us. It sucks.
If it turns out to be a WiFi calling issue (another one including the MMS-WiFi issue), then it will be a deal breaker for me because the WiFi calling is a must have inside my home.