So the next thing we need to look at is the source of the call.
Be sure your SMDR is turned on in all gateways.
Have your agents let you know when it happens, time, date and what it says on their screen (such as caller ID, trunk name, anything)
Then check your smdr logs for the record(s).
Once you understand the source you may have a better idea of what may be causing it.
Worse case you may have to call the number that called in to see who/what placed the call.
Ralph