If the call is going through the gateway, you may want to check out this doc I wrote up a bit back. It will help you narrow down the problem for PSTN calls.
Most of the CIPC users are home or teleworker types, and the most common place to look is the 800 series router that is commonly at home. You'll want to enable hierarchical QoS to shape towards the provider's speed if this is the case.
Otherwise, if it's just CIPC-IP phone calls locally on your LAN, I would upgrade the phone loads on both sides to see if it helps.
hth,
nick