K800i No access to network message

[#1]

I have a Soney Ericsson K800i and until two days ago it was working just fine. But then it displayed the "No access to netw." message and I could't make or receive any calls. I tried to restart the phone, to take off the battery, to reset the settings, to set a network manually etc etc but still nothing. And while it is searching for a network, it is draining the battery too. It was fully charged this morning,  and now it's only 25%. Normally, it should take 3-4 days to reach that power level.

I should mention now that i'm in roaming in Munich, Germany since three weeks, and my phone bills are payed on time, so it should't be an mobile operator problem...

Any ideas?

Thanks

[Martha-]

Try your SIM in an unlocked phone (that is known to be functional) and see if you have access to the network.

[#3]

Thanks for the answer!

I just tried that with a colleague's phone and my SIM didn't work in his phone, but his SIM in my phone did. So now I think it's the mobile operator's fault...