If you have a level of latency and/or jitter in your network that is damaging to VOIP traffic, will adding more bandwidth fix the problem?
You have a couple options. More bandwidth is certainly one, However, that in and of itself may not solve your problem. I would first check to be sure that all of your devices understand and can forward traffic based on the equality of service rules you have set. You may also want to move your phones to a separate VLAN so that they are not competing with the network traffic on that segment.
Next, make sure that all your ports are operating at the speed expected and have not auto-negotiated down or been...
manually set to a lower speed or to half duplex.
To determine what you would need in the way bandwidth – you can perform some erlang calculations. The site Erlang has a nice calculator to help determine if you have enough bandwidth or not.
Dig Deeper on VoIP QoS and Performance