The amount of bandwidth your specific implementation needs depends on the number of concurrent calls during your peak hours you will need to support and the CODEC (compression method) you intend to use for your SIP trunk. IPComms supports both G.711 & G.729 CODECs.
To keep it simple, use this formula:
Peak Capacity * Codec payload = Total Bandwidth required
Peak Capacity is the total number of simultaneous calls during peak business hours. Do not include inner-office calls, only calls that will be traversing your external broadband connection.
G.711 = 87Kbps per call
G.729 = 31 Kbps per call
Here's an example:
1. You have no more than 10 calls during your busiest calling period
2. You plan to the G.729 codec
Calculation: 10 * 31Kbps = 310Kbps needed during peark hours.