I had a similar issue on my n900 here in the UK and im on the 02 network. This occurred last week, I had my 3G switched on and was signed into msn via the conversations plug-in. that is all i had on. I drove over to mate house an i tried makin a call but kept coming up "call could not be established" error. I tried calling diff ppl but same result. As soon as i disconnected the 3G web, it was working fine!
Anyone know why this happens or any way to fix it........??