Xperia X10 Mini/pro :: Android Error - "SIM Doen't Allow A Connection To This Network"

I purchased it new. It worked fine for a week or so, then I switched to another network operator. When I try to connect automatically it says "Registered on network" but there is no signal. I changed 3 sim cards so far. Still the same. When I put the sim card in another phone that also works, the other phone manages to find the signal. I put back the sim card in my Xperia, it works fine until for some reason I have no signal and the circle continues. I love my phone, I want it to work, with this provider. I am very annoyed. The network provider doesn't know what could it be, they blame the phone. But the phone works with other providers with no problem so I assume it is something else.I did a factory settings restore, I checked for updates.