I worked for a small cellular carrier about 7 years ago. I would routinely test handoffs from our network to the network adjacent to us. Calls can and do hand from one network to another. The company I worked for billed a call as it was originated. I would make a call at home, drive halfway across the state (though another carriers network) and back into one of our service areas, the bill at the end of the month would show a 2 hour call in my home area, no roaming. I received the same bills as a regular customer, the only difference was instead of paying it I would send it to my supervisor and all the charges mysteriously disappeared :) And they certianly hand from cell to cell, I would often drive 400 miles in one day testing handoffs between cellsites while talking to the performance engineer all the while who was watching my signal stregths in the switch and making tweaks to the system to make it handoff at just the right time. I don't miss that job as I hate being in a vehicle.