Even though it may seem simple, there are likely many moving parts between the PureCloud Voice and the local PBX.
I do agree with
@George Ganahl, it is likely with the local carrier's last hops/last mile into the PBX. It will be essential to have that carrier monitor traffic live while testing to identify possible issues in their equipment. This is the most likely source of the issue. Now, getting the carrier to join the fun is usually the biggest challenge. 😉
Another possibility is the local PBX's ability to absorb the traffic. A traditional PBX, originally designed to handle non-call center loads, could also be overwhelmed and cause processing delays(clipping).