I would go to your carrier... Get specific call examples with date, time, caller's number, number called, and the duration of the call (try to keep over 10 seconds)... Let the carrier know you can repeat it consistently, turn then in as soon as possible. If it get's beyond 72 hours or so it makes finding the call or determining the problem much more time consuming.
I work for a rather aggressive phone company, we not only have premise based systems, but are a SIP hosting provider as well and now spend 95% of time on the hosted side of the house... If you can give them specific information for specific calls, something can likely be done. At this point you have no idea what the problem is, they can likely narrow that down.