Off to America in a couple days, and while I don't plan to use my phone i'll take it just for emergencies. Do I have to activate anything? (I'm on o2 with a Sony Ericsson K750i, which is a Tri-Band phone)
Also, if I'm using my English mobile to call a USA cell phone once I'm in the USA do I have to put the country code on or can I just ring the number as it is?
If it is TriBand you should have no problems. I did have a Motorola once that you had to 'search' for network once you had landed but all Nokias just connect immediately.
Suggest that you have all you phone numbers stored in international format always so that means you can always dial them irrespective of where you are in the world.
Go and make sure they all start with +44 and then leave out the leading '0'.
In USA you will still have to dial internationally to get a US number - so that is +1 before the number.
If you are going to use an O2 phone abroad i would phone and tell customer services, eventhough you don't have to. When I went to Tenerife in January, they cut me off for abnormal usage, as it was being used abroad.
So just tell them, just in case
I am on Threepay (PAYG) and they quoted me �15 to set it up for USA use. So I went to Walmart and bought a phone for �10 with credit on it. Still got it for next time.