Audio drop-outs or “stutters”
The symptom is audio is sounding like it’s skipping some frames.
Check by looping back microphone to speaker: check whether the symptom is observable when looping the microphone to the speaker locally.
Check for dangling call in PBX. A dangling call is call that is left active in the PBX because previous (pjsua) application has terminated abruptly.
Try to enlarge
PJMEDIA_SOUND_BUFFER_COUNTvalue by setting it in your config_site.h. Increase it to, say, 16, and see if it fixes the problem. But be aware that enlarging this buffer will increase the audio latency, so find a minimum value where the sound quality doesn’t break.