Mind you, if you're going to use one sim card and take it abroad, you must have a way to increase your airtime . . . So you have to estimate beforehand how much calling time you require and buy that before you leave the country.
I think one of us will bring a prepaid "international" SIM card that can be used all over for a fixed rate. And one of us (probably me) will pick up inexpensive SIM cards in each country we visit. That way, we'll have one consistent phone number for calls from family at home, and another ever-changing number that will always be "local" in whatever country we're in - that way the friends we're visiting in that country can call us on a local number. Now that I know I can pick up SIM cards once we arrive, that seems absolutely like the easiest thing to do.
Thanks for your help!!!