Voice calls & text messages don't use data, those are charged at the rates set out on https://ee.co.uk/help/mobile/roaming/roaming-costs > India. Call & text add-ons are also listed here.
There is a PDF somewhere on EE's site that lists the various data-roaming add-ons, but I can't currently find it. https://ee.co.uk/help/mobile/roaming/travel-data-pass lists some of the rates.
Regardless of which billing add-ons you choose, remember to text ROAMING to 150 before leaving the UK to enable the free roaming facility. Failure to do this will prevent your phone from connecting to foreign networks.
