man, there's a lot of variables along with the similarities.
One obvious question: Is it on every call, or only occasional? Is there any commonality with the type of call they're on (e.g. talking to someone on a cell phone, VOIP phone etc)? I can get pretty weirdly variable call volumes on my residential land line if I'm talking to people who on crappy home VOIP phones.
A couple of things to try:
Put line appearances on the phone and answer line appearances and see if it's tied to a particular line (or possibly trunk port on the combo card)
If it seems to be a single line, you could unplug the co line and make a call with a buttin, see if the issue is still there.
Try moving them to a different port, got any DS8's in the system to move them to?
Got to work to isolate it more, somehow.
GB