You only need a tiny amount of bandwidth (connection speed) in order to use hosted PBX effectively. Each channel (one call) requires only 80Kbps (0.08 Mbps),
or using compressed codec even smaller – 0.03 Mbps. So even with 24 Mbps Internet speed it will be able to perform at least 300 simultaneous calls. However,
quality of calls will highly depend on the reliability of your
internet connection,
so it does not have to be very fast but it must be reliable and stable.