Hi,
In my experience with no audio issues, it is related to network routes.
Validate ALL the chain, I mean check with pings and traceroutes all the path from phones or softphones) to Media Servers (and MGs), and of course the SBC. It is usual those elements are on separete VLANs.
Some times we have seen path "brokens" or no route at all
And some times the headsets are not configured well on laptops
Hope it helps.
regards