Another option that I don't think anyone's mentioned is to buy an International SIM Card for your mobile phone. There's a few websites selling them but the cost/tarriffs differ on each one so it can be a little hard to compare them.
I've just bought one for £16 to take to Turkey with me, this includes £10 credit already loaded on the SIM. On this particular SIM it costs 25p a minute to call a UK Landline from Turkey and 39p to text to a mobile in the UK. Also there's no charges for receiving calls or texts on your mobile. The main reason I bought one was that it would be easier for someone to get hold of me from here, if they needed to. If someone wants to phone me from a BT landline it'll cost them between 10p-20p a minute.
As long as you use the SIM at least once in every 9 month period it won't be locked and you can carry on using it whenever you're away (it can also be used in the UK but wouldn't be cost effective). You top up the SIM over the phone using your credit card.
I've not mentioned which site I bought the SIM from as I'm not sure if that would be classed as advertising and if that's allowed. Money Supermarket compares most of the more established companies, which is where I saw the site that I bought mine from. It's the first time I'll be using an International SIM so won't know how good the service is until I try it but a couple of people I know have used them and found them very reliable, cheaper overall compared to buying phone cards and more convenient.
Josh