Depends. If you send all the calls to the same destination and use the “packet saver” option ( it is a software option that you could buy), the bandwith used will be something like 30% less.
Calculate a minimun of 10-12 kb of bandwith per active channel.
I will not feed it less than 512 kb, that way you have a “safety margin” when your provider is congested.
If bandwith is a premiun at your location, consult about the packet saver option with Quintum.