I have been roaming overseas this week and at one stage, only briefly, a new menu item appeared in my Phone application's "options" menu. It was "Second Line" (or very similar).

Its appearance confirmed my suspicion that the phone supports such capabilities even if my service provider doesn't. It's gone now.

Does anyone know more about it, including why it appeared on my handset? I was using my regular SIM but was drifting in and out of (Thai) networks. I tried manually connecting to all of them but couldn't recover the menu option.