I am trying to forward private lines to their mailboxes. I have cfna set and timer is 4 rings intercept is programmed for their ext. but it still goes to main message.

when you say intercept where to, you should have each private line ring dil to extension, then extension fwd n/a to mailbox
If you intercept an extension or line to voice mail it will go to main greeting

I have the disabled the intercept, the line rings on his phone and the cfbna is set to 115. It just rings and rings.

normal day ringing and DIL are 2 different things. Not always but vmail is usually 165.
May be time to bring in someone that knows Panasonic.

kjwiewall , it sounds like the voicemail may not be digitally integrated, (DPITS).
From any phone dial intercom 165 and see what happens.
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MrG