The telephone system needs a very high input level of 4V p-p for the music source. Can your music source provide such a high level?
The very simple wiring shorts the two channels together which might blow up the music source.
The instructions for the 3.5mm jack say that pins 3 and 4 are not connected to anything so why do you ask? Pin 1 is the common shield wire, pin 2 is channel B and pin 5 is channel A.
The instructions given in the pic are to connect pins 2 and 5 of the socket together, leaving pins 3 and 4 unconnected. This adapter is intended for use with a normal stereo lead terminating in a stereo plug.
However, I'd be more concerned about possible damage to the stereo source by high voltage (typically >50V) present on the POTS phone line.
The extremely simple wiring stupidly SHORTS the channels together to convert stereo to mono (and maybe to make smoke) instead of using a simple mixer with two resistors.
The source is stereo (not mono) and uses a stereo 3.5mm plug that plugs into the headphones jack. The level will probably be too low.
If you have a mono music source then it has only one channel and a mono 3.5mm plug can be used. Then pins 2 and 5 of the RJ45 plug are shorted together because we don't know which one is used on the telephone system.
The extremely simple wiring stupidly SHORTS the channels together to convert stereo to mono (and maybe to make smoke) instead of using a simple mixer with two resistors.
This is one of AG's personal bugbears, it's standard practice to short them together to make them mono, it does no harm whatsoever, the units are designed to be safe from any damage in doing so.
But back to the thread, there appears to be some confusion about exactly what the OP is trying to do? - I'm presuming it's connect 'music on hold' to a phone system that's designed for that purpose?, so it won't be connecting direct to the phone line, but to some specific interface. As it was mentioned to be 600 ohm, it's probably vis an isoltation transformer for safety purposes?.
No that diagram does not show a RJ45 connector. Its showing a switched stereo jack.
Lets start with what connector is on your phone system? A RJ45 or a phono plug?
The instructions admit that you might not get 4V p-p from a line-out music source so to use an amplifier to boost the signal.
But the instructions do not show an mono amplifier, they show a stereo source with its outputs shorted together.
Maybe you can short the headphones or line outputs of some products together to feed a mono amplifier, but don't short the outputs of a stereo amplifier together. Then the output of the mono amplifier connects to the RJ45 jack.
Hardly 'some', it would be difficult to find one where you couldn't, I've never heard of one that couldn't - although I suppose if you designed it deliberately to be damaged, it 'might' be possible?.
but don't short the outputs of a stereo amplifier together.
You need to know what pins of the RJ45 connector on the phone system to connect too. Do you have any documantashoin thats tells you? The document in the first post does not tell you this. Andy