How to improve Audio quality in sip phone

How to improve audio quality ?

These are some key points
Good Bandwith

WideBand codec

Wideband VoIP only works when sent VoIP from end to end. With the prevalence of existing PSTN infrastructure in our lives, pure end to end VoIP links still make up only a small percentage of communications traffic.

QoS, You should use a router that prioritizes VoIP traffic over other types of internet data.