Calls to India: 1.5p/min
Take a tesco value sim card with double time ( 10 pounds card 20 pounds airtime)
text 'card' to 88022. They will text you a pin and the 0207 number. The pin will automatically topup once your credit expires. The charge is 3p/m. but since you get double the talk time it will cost you only 1.5p.
Hoever if you will use 0800 number the net cost will be 2.5p/min.
I have used it for last 4 months. voice quality to Delhi Landline or mobiles is excellent. Additionally it tells you about your balance after every call so you are aware of your usage.
There are a few deals posted from tesco for the mobile sim card. the link I have provided is the link to the card. The link to the sim card ishttp://direct.tesco.com/promo/?id=S00005977
Hope this will be of use to someone. If you find a better deal post it here please otherwise if you feel this deal is useless, please do say why do you think so.