I still have to try to isolate exactly which component(S) have failed in an older Prostar816 Plus. No matter which phone is on the end of which line... or if there's only one phone, and everything else disconnected except the incoming CO lines... the "HOLD" function fouls up 3 of the 4 incoming CO lines. With (or without) a caller on line... press the "HOLD" button... and lines 1,2,3,4 will be unavailable for several seconds.
Line 5 seems unaffected. Usually. The longer you spend pushing CO line buttons... the longer it takes to recover. Best bet if you forget and hit "HOLD" is to just sit and wait for about 20 seconds. (If you had a caller, the line is dropped. They hear "click"... and then they're gone.)

I've played all the moving phones around stuff, disconnected everything, reset and reinitialized and re-entered all the programming I can several time to no avail. The only manual I have is the blue 82-page Installation Manual. Which seems about useless from this point on.

It sound to me like a problem on board somewhere, but I have no idea at all how to chase it down any farther.

Halp!
(ref the question "Fried Chips" on the other Sundance BB under Samsung for a few more details.)
Jason