to terminate any VoiP call to telco you need a connection to telco/PSTN.
For international calls most of these voip co have tied up with some telco and have took the,PRI/BRI lines to terminate the calls to that telco which then forwards the call,to other telco through interconnections. most of these voip co have there sever in diffrent parts of word and mostly use sip/IAX/cisco skinny with optimised codec like g729 and others,which consume very less bandthwidth, what these co's charges form the customer about 50% goes to these telco's for providing termination.
now in india where GOVT OF INDIA has disabled/not allowed the viop termination for the calls origanting from india for all the telco so they will not terminate the VOIP calls DIRECTLY on there PSTN(give you interconnection)
but if you need bulk calling (using more than 5000 min)there are always cheap and econmical ways of setting up your own network using open source
SER or IAX trunks between the cities using asterisk and then routing the calls through PSTN