The organization(s) that migrate from legacy PABX system (traditional phone system) to IP based PBX (Private Branch eXchange), they require a very carefully planning before they deploy the VoIP.
They need to ensure the QoS (Quality of Service) for VoIP in your organization.
Analyze your existing LAN (Local Area Network) and find if there is any congestions /delays or unnecessary broadcasting or bursting.
The IP address that you plan to assign to the sip server ping it from several points/node in a network and find the ping statistics and you should look for
High latency/jitter will cause a very bad effect on VoIP quality and you will not be able to hear clearly.
Check your network switches and theirs interfaces. All network switches need to have same interface capacity. It should not be the case that one switch you have is 1000 Mbps and the next switch is or after a next or next to next you have 100 Mbps.
This will cause network bottleneck.
Create the VLAN (Virtual Private Network) to separate the data and voice traffic this will give a very good mileage to your voice traffic as they have separate network and you will have greater quality.
If you don’t afford the VLAN (as it’s require manageable switch and relatively expensive as compare to non-manageable switch) then you should deploy all the 1000 Mbps switches in your network that will give more local bandwidth to your voice traffic.
If you require the HD (High Definition) audio quality then install g722 or g729 audio codec. This is an optional but it will play a critical role in conference call or phone meeting as you will be able to hear clearly over the phone.