What they do is, giove you a VOIP gateway which you connect between your router and computer. The comp is connected through the ethernet port and a telephone (RJ-11) jack connects to a regular phone.
The gateway is assigned a local US number. So, in effect you have a local US number which can make and receive calls.
You just use it as you use the BSNL/MTNL landline. No difference at all in voice quality and the gateway does a QoS on the line. Even if you max out the connection with bit-torent, the gateway will reduce the bandwidth to the computer by ~3-4KB/s (which is all that is consumed by the VOIP connection).
You receive and can make calls like a land-line with the only difference being that the voice is routed through the internet.
Also, allmmost all of my friends and relations use my above mentioned VOIP service.