If you use a UK mobile to make or receive calls in India it is very expensive. You need to check on your suppliers web site but it is likely to be about £2/min.
If you want to make or receive calls it is better to take an unlocked phone and buy a sim card in India. The card costs about £1 and then you top it up with credit (I topped up with 1000 rupees).
To buy a card you need a photocopy of your passport and proof of where you are staying (pay your hotel bill and take the receipt). There is some form filling and as it is India it will probably take about 30 minutes). To make calls to the UK will then cost about 10p/minute (or less). You can give your Indian number to people at home but it will be expensive for them, but you could phone them back. You do not pay for incoming calls
If you move to a different region in India your phone will still work but you probably will not be able to top it up.