Hi - 120 PSTN calls through 4E1 PRI lines. The number of IP Calls depend on your WAN bandwidth. And if QoS is implented how much bandwidth is allocated for voice traffic. PSTN calls will be routed via E1 lines unless you have a SIP Trunk as well to ITSP. IP Calls will be routed over WAN.
Roughly, g729 uses 24k and g711 uses 80k. You can divide it by your allocated WAN BW and get no of calls. For example: 240k bw will allow roughly close to 10 g729 calls.
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