Reliance is cheapest IMHO.
Stay away from cards that promise X minutes for $5 or so. They will usually have hidden charges and idiotic way of counting minutes.
If broadband is available at home, then buying unlimited vonage/ skype plans and setting them up in India is the cheapest way to go.
vonage and other SIP services works out easier since a PC+Headphone is not required. You can plug existing cordless/other phones into SIP box. |