I live in Australia and what I do when I travel overseas is divert my home cell (we call it a mobile (phone) here) directly to voice mail so you do not get hit with expensive call/data roaming costs and buy a prepaid SIM card (which has data/internet) for the country I am travelling in. In Australia you can buy prepaid SIM cards at most GAS (Petrol) Stations, 7/11 or stores like Coles, Woolworths or Safeways. With the iPhone they take a micro SIM card which is smaller than other SIM cards so I purchased a micro SIM cutter ebay, basically it is like a hole punch but you put the regular SIM in it (make it is in the correct way) then press and the micro SIM is cut out of the regular SIM. Not sure if you can buy iPhone SIMS on prepaid here cause I have never done it. Mobile Phone Co's include Telstra (probably best coverage for most of Australia) Optus, Vodapone and heaps of resellers for these Co. Telsta is best for coverage.
Hope you have a great trip.
PS I had a friend that went to NZ and took his mobile with him, did not make many calls over there but 1 of his kids was playing with his phone playing games and he got hit with a bill for $3,500 data roaming, the kid was playing some game that accessed the internet!!!!!!