For g729, about 20kb per active call.
To be absolutelly sure, calculate 22 to 23 kb of bandwith per active call on g729.
For a DS3 (672 channels)you will need a BIG switch.
Most regular gateways go up to a handfull of T1 ‘s only.
Above 10 T1, you are talking only a handfull of very expensive vendors.