I too use my UK Three SIM card in most of the countries I visit, with their Feel at Home package – it works in various European countries, USA, Australia, Hong Kong, Indonesia, and others. Annoyingly (for me) not in Malaysia or Thailand, but I have a PAYG SIM card from Hotlink (Malaysia) which I can top up periodically from here via their app, which stops the number expiring. Hotlink seems to offer reasonable roaming rates for most other Asian countries (I think I paid 38MYR – – about £6 – for 1 or 2 GB last time I was in Thailand, and the same in Cambodia. I also came across a thing called Flexiroam (www.flexiroam.com) – a microchip you stick over your existing SIM card to let you use a free data allowance overseas. I haven’t used it myself, and I’m not sure what the position would be re ‘interfering’ with your SIM card if you’re in-contract, but it may be of use to some.