Background:
POTS from CO not off of an IAD or any such electronic device.
Works perfectly at demarc with house wiring removed.
Extended from demarc to 66 block. Works perfectly on both sides of the 66 block.
25 pair cable extends upstairs (100 ft +/-) and is punched down on a patch panel.
All other POTS lines via same facilities work perfectly at patch panel.
The one in question when you go off hook takes 15-20 seconds to return dial tone.
Changing the port and swapping with a known good port (good still good on the "offending" port) gives same results.
Voltage doesn't change from demarc to patch panel

Any thoughts on what I should be looking at next?