here in bangalore i use spice and they have a offer now you pay Rs 19 per for every sat and sunday in the month and then call rates are Rs 1/- to any where in US to any number be it mobile or landline.
so may be there are some thing like this there too, else you can use country specific cards which are i think cheap, else if there is Iway near your place call from there its cheaper from the normal ISD booths.