Not sure if this is common knowledge but I just got off the phone from Three customer services to try to avoid paying my DD whilst I'm away in Australia for 12 weeks.
I was advised that i could take a suspension which means my account wont be charged for the time im away. In return all you have to put up with is an effective extension of the contract when I return. For me that means putting back my upgrade date from January to April and my contract termination date from May to August - no skin off my nose as I'm happy with my tariff!
I guess the other major mobile providers will offer the same service if you ask.
Hope this helps anyone who's moving abroad temporarily :)