Excuse me the delay in answering. I have configured "Autodisconnect"to switch to 3G by connecting to the mobile network and back to 2G at the end. If I'm on a call using 2G and during the call the N900 is connected to the network ... the call is dropped. The same applies if I'm talking to someone on 3G and N900 disconnected from the 3G Internet connection, changing it to 2G.
dbus-send --type=method_call --print-reply --dest=org.freedesktop.Telepathy.ConnectionManager.ring /org/freedesktop/Telepathy/Connection/ring/tel/ring org.freedesktop.DBus.Properties.Get string:org.freedesktop.Telepathy.Connection.Interface.Requests string:Channels