Never ever use rubbish three network. Vodafone is the best for network but expensive like Docomo. O2 is the best for freebies and offers an discounts. o2 is like Softbank but more flexible. Giff Gaff is rubbish as you only can charge online only using o2 network. Lebara/ Lycamobike is ok for certain international calls but not useful as you use credit faster talking Internationally. Virgin gives great deals but only for 12 month contract users. T-mobile and Orange is now EE best for calling if you plan on living in London. Not do great on signal for data. Three is the worst ever network, chinese go for this network because they say its unlimited internet. it's the worse network to have. You must live or work in Central locations of a town and have modern or chinese mobile for best coverage. If you are saving money you can not use this network as you can not get no network unless you are outside or in a middle floor apartment or house.

Thank you Calvin Lee. There are many carrier!! I know Vodafone. About 10 years ago Vodafone was there in Japan.

In the UK, there are EE, Three, Giffgaff, Lebara, O2, Lycamobile, Vodafone and Virgin mobile. I'm using Three.