In the USA, we would normally think of the iPhone with its AT&T Carrier association. When traveling abroad, there are many other carriers that are available. In the screen shot seen here, you will see the multiple carriers that one can select, or have the iPhone select one automatically.
iPhone Carrier Settings Screen
With the Automatic selection, the iPhone will actually bounce between carriers depending on strongest signal or closest tower. This could mean several locations for subpoenas to be issued for important case data. Be cognizant of where an iPhone has been when investigating and analyzing this device.