This used to happen rarely (and appeared to be linked to a particular incoming caller), but my boss says it's happening much more often.

What happens is: We're on the phone talking with a customer, when another line rings. Suddenly the phone we're talking on hangs itself up, interrupting the call and insulting the customer (and, of course, when we pick up the new incoming call, it's likely to be a telemarketer).

Any idea what might cause this?

We have 3 voice lines (via Cox Cable), 10 phones in the system, most are 48-key. I think it's a C-Class system, we've had it for 5 or 6 years.