I don't even answer calls when in Australia if I don't know the number calling. I won't answer calls overseas, even from numbers I know, for the same reason you mentioned. I'm not paying extra roaming charges simply because someone else called me.
If I get a SMS that says a flight is cancelled, I'll act on that message. If they try to call, they won't get through.
I've only had 2 flights cancelled (not simply delayed) during my travels. Both on QF.
The first was a domestic flight. They moved everyone to an earlier flight. I didn't get the message. At the airport for the original flight, they claimed to have left a message, which isn't possible as I don't use voicemail, nor do I have it turned on. They didn't even call the number that was listed in the booking.
The second was for an international flight. I started getting SMS messages about it. I was sitting in the MEL F lounge at the time and could deal with it directly with ground staff.
