On my last two installs, I have had a similar issue with incoming call loops. Both used analogue trunks.
The first install was set to ring a hunt group, which is unanswered recalled to a CRACRA message and this timed out to the general mailbox voicemail. One day after install the general mailbox has 355 empty 5-second messages from the same caller id and seemed to be receiving a call from them every 2 minutes. It looked like something internally was 'redialling' the CRA to me and then directing it to the timeout 'voicemail' I eventually reset the system as we were unable to stop it any other way.
Then on a different install yesterday - as soon as we connected to the analogue trunks we received a call to line 1 and the answer hunt group. Then after hanging up, the same number called line 1 again and again, ringing the hunt group phones but there was never anyone on the line. This time we unplugged the line 1 cable and twisted it together to make it busy. Then when we reconnected a few minutes later, it worked properly and has worked since.
I wondered if this could be the same issue - either with my programming or maybe even the provider ( same trunk provider for both) It seemed to happen on the first incoming call when the live trunks were connected.
Anyone else ever experienced something like this?