PSTN inbound rings for a while, stops, rerings with no CID

I have a single PSTN line coming in via a pcie dahdi card. Everything works great, except for one minor annoying problem… If a call goes unanswered for a while, it stops ringing briefly, then starts again as a new call, but with no caller id this time.

This also means any missed call lists are coming up as 2 calls every time. One with caller id, the next as “unknown”.

Any ideas where this might come from or where to start looking?

On the pstn a ~500ms drop below 3v is considered a cpc disconnect. So if you are using an Ata, cable modem or some electronic device that drops below 3v for ~500ms then you get a new call

Line comes in, ADSL filter splits voice to the card and ADSL goes off to the modem (Cisco HWIC-1ADSL in a 1841). Nothing else is connected to the line.

We do also have a telco side voicemail on that line if that might be a trigger. Id have expected the call simply to have ended as far as that goes though.