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¢/min freecall
3.3¢/min calleasy
3.3¢/min webcalldirect
3.9¢/min net2phone
4.2¢/min smsdiscount
5.0¢/min internetcalls
5.0¢/min nonoh
5.0¢/min voipdiscount
5.0¢/min voipwise
5.5¢/min freshtel.net
5.9¢/min pc-telephone
6.0¢/min mywebcalls
6.7¢/min 12voip
6.7¢/min justvoip
6.7¢/min poivy
6.7¢/min sparvoip
6.8¢/min gtalk2voip
7.1¢/min vbuzzer
7.5¢/min webphone
7.9¢/min yahoo
8.0¢/min click4
8.0¢/min callserve
8.0¢/min hotfoon
8.2¢/min blasterphone.net
8.4¢/min netappel
8.4¢/min sipdiscount
8.4¢/min voipbuster
9.1¢/min voipstunt
9.9¢/min iconnecthere
9.9¢/min lowratevoip
10.0¢/min phoneopia
10.6¢/min skype
11.4¢/min sipnet
11.6¢/min voipcheap
13.8¢/min wengophone
14.6¢/min gizmo5
16.1¢/min peneo
19.9¢/min voipbusterpro