Did a CLEC cutover a few weeks ago, and everything was working well. Customer called Friday and said external paging and ringing over page was not working. Went to the site today.

Paging horn works, I can take the RCA plug out of the page port and plug it into a music output and it plays through the horn. Plug the RCA back into page port and dial [DN]#39, display says external page, but I get nothing. Also tried #35-#38 and got nothing. When I dial #30 I get "All Call Busy."

They have a Valcom 1030 and a VP-624. He also said that ringing came through that, though there is no interface for generating a warble tone. I re-did all the button programming at the cutover, and I am thinking they may have been using night ringing with the ring over page(program 78), and I did not program at night key with their new layout. That would not effect voice paging would it? I know the paging was working after the cut-over and reprorgramming, because we tested it.

Any ideas?

You are correct that you must have a NT button programmed on a phone for night ringing. No external interface is needed for ring over page, but the system has to be in night mode.
You are also correct that not having a NT key has no effect on external paging. As far as the external paging, you seem to have verified everything down to a bad paging port. I've never had a page port go bad, but anything is possible. One thing I would still do is double check the wiring on the RCA plug for a bad connection.

A quick swap with a spare, defaulted KSU would verify your findings.

I have thought of doing the swap, I may do it tomorrow. I actually used that RCA plug to connect the paging to music on hold and it sounded fine, so I am pretty sure it is not the plug.

Put your test set on the paging port and make a page. That will verify the paging port.

Sadly, I tried that too. Never had a page port go bad.

Been ages since I've worked with the DK's, so this is a bit of a shot in the dark with a blindfold on.

Do you happen to have an expansion box with a PIOU (?) in it - that could be used for paging. I forget the exact card number, but our DK40 used one version for remote programming, but I remember it had a single paging output which would have had a twisted pair connected to screw terminals. Our multicabinet DK280 had a different version which had multiple paging zones and an amphenol connector.
Both cards would have required a transformer to convert from 600 ohm twisted pair and connect to your RCA plug.

Question for the experts: what programs are used to disable the internal page port and use the PIOU?