While hunting for a VoIP service to call India, I found a fair variety of services that I'm sharing below.

FreeCall appears the cheapest when calling India, at 2.5¢ per minute to a land line. I'm listing the rate from London to Chennai below. I'm not sure of the difference in voice quality between these. The only one I've tried is VoIPDiscount, which is not too bad. As a benchmark, remember that Reliance offers a calling card at around 7.3¢ per minute.

(Incidentally, if you wanted to call the US/UK, there's no reason why you should use your phone. Calls to the US, the UK and parts of Europe are free with most of these services.)

2.5¢/minfreecall
3.3¢/mincalleasy
3.3¢/minwebcalldirect
3.9¢/minnet2phone
4.2¢/minsmsdiscount
5.0¢/mininternetcalls
5.0¢/minnonoh
5.0¢/minvoipdiscount
5.0¢/minvoipwise
5.5¢/minfreshtel.net
5.9¢/minpc-telephone
6.0¢/minmywebcalls
6.7¢/min12voip
6.7¢/minjustvoip
6.7¢/minpoivy
6.7¢/minsparvoip
6.8¢/mingtalk2voip
7.1¢/minvbuzzer
7.5¢/minwebphone
7.9¢/minyahoo
8.0¢/minclick4
8.0¢/mincallserve
8.0¢/minhotfoon
8.2¢/minblasterphone.net
8.4¢/minnetappel
8.4¢/minsipdiscount
8.4¢/minvoipbuster
9.1¢/minvoipstunt
9.9¢/miniconnecthere
9.9¢/minlowratevoip
10.0¢/minphoneopia
10.6¢/minskype
11.4¢/minsipnet
11.6¢/minvoipcheap
13.8¢/minwengophone
14.6¢/mingizmo5
16.1¢/minpeneo
19.9¢/minvoipbusterpro