Lets all agree there is no butt set connector that taps onto a 110 block and that is because different manufacturers transmit the 1s and 0s at different rates and different patterns and different voltages and have different characteristics when a 2nd phone is plugged onto it.

That said, there is one rational way to do this.

Get the 110 to modular adapter that several people sell. You first disconnect the phone at the far end. Then with the same style phone in hand, you can move from the ksu to the first, second and so on connection points.

At some connection point the previous connectiion works and this one does not. There or somewhere inbetween lies the problem.

With a Samsung for example, you don't want to carry a lightweight 8 button phone and plug it into 110 connection points for a 24 button phone because you will reprogram every phone you touch that is not an 8 button phone.

This would not happen on a Toshiba but the tech should know which ones would work and which ones must only be tested by the identical style phone.

If you unplug the far end set that has trouble and clip onto the 110 block with your modular adapter and use the same style phone, you are jack cool.

Hey look at my Mitel SS4150 test set, it looks just like a real phone but I can do diagnostics with it.

I tried to shorted Ed's idea, but I think I went in reverse.

THE Bracha, old blond specialist in Rube Goldberg solutions.