I have a site which uses a 3rd party dispatch board to answer and transfer calls. The SV9100 hands off the calls via SLT ports.

The problem is 40% of the time the system will skip over the 2nd ring. When listening with a butt set. You hear the first long ring followed by the caller id being sent. Then when the 2nd ring should go through you can hear it try to kind of ring but never sends any voltage and almost sounds like it's being shorted. The other 60% of the time it works fine.

The issue is, if they don't answer the call quick enough. The dispatch board thinks the call has hung up, then when it sees the next ring it thinks it's a new call and receives no caller id.

I've removed all the wiring from the equation. It still does it with only the butt set connected. They have all IP phones so there are not any other cards in the system.
I've replaced the chassis, verified it was grounded, replaced the SLT card and changed the ring cadence to different ones. No change. NEC says there is nothing they can do. If i disable sending of the caller id on the SLT port the second ring always works.

Needless to say I'm it a loss.