Mobile Phone Roaming Charges
There are always additional charges to pay when using your mobile abroad. Even though you are roaming, your mobile phone number stays at home. This means that all outgoing calls will go direct to their destination, but all incoming calls must go through your home network.
Incoming calls to your phone number will be forwarded to your mobile, wherever it is. The caller will pay for the UK leg of the call (as if you were still in the UK), and you will pay for the international leg of the call (from UK to whichever country you are in). This is because your caller will not necessarily know that you are abroad, and so cannot be expected to pay for an international call they are unaware they are making. Consequently, you should look carefully at roaming charges when choosing a mobile network if you plan to use your phone frequently in other countries.

