Two options here, Buy sim cards or each country, or buy a global sim card.
In Thailand you can buy a sim card for the True Network from any seven eleven store for only 150bht, the minimum top up is 100bht and calls to the UK are about 5p per minute when using a cheap access number. You can also phone this number for only 1p a minute from any BT phone in the UK by dialing a cheap access number, an example is
http://www.telesavers.co.uk/ ... You will need a passport or ID when buying a sim card in Thailand, due to the amount of bombs going off in the south triggered by mobile phones, the government has made it compulsory to buy with ID and register the card (takes about 2 minutes to register, and the card will be working within the minutes. True gave me the best coverage when I was in Thailand last year, but things might have changed since then.
In China, China Mobile is said to be best for coverage all over China. The sim cards can be bought from almost any local store or one of the numerous accessory shops you find all over china. just ask for "Sim Ka", again calling china is only 1p per minute from any UK landline (Cheaper to phone a chinese mobile than a UK one!!!)
Philippines, Same again, sim card for about £2-£3,, calls a bit more expensive about 6p per minute.
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The other option and the easiest one is to Buy a global sim card for the phone, look at
http://www.0044.co.uk/global-sim-card.htm for an example. Calls are about 20p per minute to the UK from anywhere in Asia.