You can actually also get billed for incoming call diverts to voicemail if you just switched off your phone on a roaming network. I always divert all calls to voice mail prior to switching off the phone. This ensures that the home network doesn't try to route a call to a remote network. Even though the remote network won't find you as the phone is off it will still attempt to process the call, fail and divert back. The timeframe for how long your home network tries to find you is set by your home net.