there can be several reasons for it.
I also hope you are not on the same release as the 2015 system.
maybe it is not a random ring but analog lines not disconnecting and then ringing but by the time they get picked up there is no dialtone.
The only way to find something like this is tracing with Monitor or/and System Status
I use SSA first as it is easier to read and follow and I love the playback feature.
Then you can see what happens exactly with the call and where it comes from.
Joe
FHandw, ACSS, ACIS
"Dew knot truss yore Spell Cheque