Work out how much bandwidth you need

Each call uses bandwidth in both directions.

Steps

1. Count the most calls you have at the same time.
2. Allow roughly 100 kbps each way per call for G.711 (about 87 kbps plus headroom).
3. Multiply by your busiest call count, for example 10 calls × 100 kbps = 1 Mbps each way.
4. Keep plenty spare for other office traffic, or use QoS.