what does 1 min of VoIP usage translate into in terms of data transfer?

I am trying to work out the total cost of a VoIP service. Got the call rates but one component is the bandwidth usage. Since it works well with 256kbps and above, and both VSNL and MTNL do not have unlimited plans for 256 kb/s and above therefore the question.

Ignorance is Strength. What you don't know cannot hurt you.

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