DSX 80 with 4 POTS line. Line 1 and 2 ok. L 3 L4 , when calling in and receiver is picked up, person can talk to the other but its still ringing in the handest. Switched 1 and 3 and still doing the same. Called NEC Tech Support and they said theve never heard such before. Any ideas?

Did you reboot system ?? What happens if you go to Demarc and disconnect trunk l1 and call in on butt set ??

I've had strange issues in the past with both the DSX and the SL1100 that required me to re-initialize the system & re-program (not from back-up) to rectify the situation.

Ya I would check on a buttset. Another thing could be a shorted tail.