Yeh as CIT just said have the system and a SLT connected to the lines and see if they both ring on the phantom calls.
DACS over here is when bt ran out of pairs to a site and its used to get 2 analogue lines from 1 pair.
https://en.wikipedia.org/wiki/Digital_Access_Carrier_System Can also try defaulting to see if it still does it and restore if not.
