Determining the reason for poor call quality should be a troubleshooting exercise any good telecommunications partner should be able to get to the bottom of.
The quality of the phone system is dictated by a lot of factors. It is not usually the fault of the SIP Trunk provider, though it’s always possible!
Using a Softphone
Quality of Headset
A common practice is to implement QoS or Packet Prioritization.
This will make sure that VoIP traffic gets priority over all other traffic on the network ensuring that there is enough bandwidth for the VoIP call. If you are having issues with bandwidth, check which codec you are currently using and possibly change to one which requires less bandwidth. Find out a bit more about codecs here.
Quality of the other persons line
Aged networking hardware
If switches or hubs are being daisy chained, this may be creating a bottleneck for network traffic and it’s possible your call will not have enough local bandwidth to reliability deliver good voice quality.