The free roaming facility, which enables your phone to connect to foreign networks and thus have coverage abroad, is a one-off process.
For PAYG - ensure you've used your phone in the UK before travelling. For pay-monthly, text ROAMING to 150 before leaving the UK, and follow the prompts.
The costs vary between PAYG & pay-monthly, but can be found on https://ee.co.uk/help/mobile/roaming/roaming-costs > China. You'll receive a welcome text message when you first arrive, with details of how to buy add-ons.
