I did this before.
Keep the sim card in the phone, never take it out. Once you take it out, it recognize that and needs to get Facetime activated. In order to get Facetime activated, it needs to send SMS.
What you'll need to do is when abroad, turn the airplane mode on, and turn wifi on. This will keep facetime available and use your #.
My worry is that I know people who have used the airplane mode, assuming as you do that will prevent the problem, and for whatever reason, whether it is when the turn the phone off, or use certain applications, data has turned back on again and they have ended up with seriously high roaming charges.