There are two cases:
1. A Visited network belongs to the Home network country, then MS shall periodically attempt to obtain service on its Home network. For this purpose, a value T minutes may be stored in the SIM (from range 6 minutes to 8 hours in 6 minute steps) OR IT INDICATES THAT NO PERIODIC ATTEMPTS SHALL BE MADE.
2. A Visited network belongs to the other country, then MS will act, as Pix said