It seems that the flaw is in the cordless phone, that somehow the 5 volt power circuit is connected to the phone line circuit. THAT SHOULD NEVER BE!!AND, in the router, the phone line connection is connected to some other circuitry. One option might be a 1:1 isolation transformer in the phone line between the two.
BUT an isolation transformer may be a problem if DC continuity is used for phone control.

 

It seems that the flaw is in the cordless phone...... THAT SHOULD NEVER BE!!

I've tried a another set of cordless phones and the problem exists with them also. This second set of phones do not display an error they just don't work - no dial tone or dialing out. This to me would suggest the flaw is not necessarily with the phones.

The original phone base unit is a Panasonic KX-TGC220E, release date about mid 2016, which I believe were a quality product designed for the UK market. The newer phones are BT Essential Y release date Oct 2017; these newer phones bought about 6 years ago have never been used in anger because the sound quality was/is awful.

One option might be a 1:1 isolation transformer in the phone line between the two. BUT an isolation transformer may be a problem if DC continuity is used for phone control.

Is this here what you are referring too? If it is, would I simply cut the line cord and connect the transformer as per the diagram below?
Would swapping the connections to C&D make a difference e.g put the input and output out of phase (if that's the right term)?
How would I tell if DC continuity is required for phone control?

 

The diagram is exactly what I was talking about! If a DC connection is also required, then the transformer scheme will notwork. And as Iconsider it, probably it will not work.What needs to be isolated is the power. The phone needs to see the phone line voltage change when a call comes in.

 

I'll look to get an isolated DC-DC converter but before I do a couple more questions.

This module refers to LDO pads for more stable LDO output and Reserve LDO Output; is this something I need to be concerned about? see picture below curtesy of eBay and Technicalaim


In the picture above the device has a connection labelled Vadj on the output side of the PCB, presumably voltage adjustment. If this connection is for voltage adjustment any Idea how this would be implemented; could it be for use when LDO is used?

For efficiency and bearing in mind the load drawn by the phones is about 200mA when charging and 50mA in standby, would it be better to use a low output current converter, say 1.1 amp, as in the device in the above link or a higher output current as in this device or does it really not matter? I was wondering if the higher output current device would give a higher output voltage, nearer to the phones specification of 5.5 volts, due to the converter being under loaded and its voltage regulation being less precise at low load.

I don't know if the device in the second link is isolated and I'm going by its input wires being red and black and output wires being red and yellow that it might be, so I've messaged the seller to ask.

 

That looks like a whole lot for just a DC to DC isolation supply. Before you buy, I suggest checking the DIGIKEY on-line catalog.

 

That looks like a whole lot for just a DC to DC isolation supply...................

Sorry I'm a little confused - That looks like a whole lot of what?

 

It looks like a lot of electronics for what should be one single quite small module. So once again I suggest checking the catalog of an actual electronics component seller, like Digikey, Newark , future, Pioneer.
Depending on where in the world you might be located, some others will have better suggestions.

 

I'm in the UK, where the phone systems may well be quite different to the US's. We're way behind the US in digital (VOIP) change-over so some of what I've said may not be relevant over there.

 

I'm in the UK, where the phone systems may well be quite different to the US's. We're way behind the US in digital (VOIP) change-over so some of what I've said may not be relevant over there.

The UK may not be quite as fast to jump onto each FAD, possibly. Thee "benefit" that we get from all the latest technology is two dozen calls a day, that have spoofed caller IDs, trying to rip us off with medicaid frauds and similar.
As for the phone connection, correct functioning depends on having both DC and Audio arriving on the red and green pair. So the isolation that you need is with the phone power source. It seems that the phone line connection is linked with the phone power connection. That means that your wireless phone base needs an isolated five volt source.

 

Ok thanks

 

It might work to power the cordless phone base with a separate battery pack that was charged thru resistors from the main battery backup system. Resistors in both positive and negative to provide isolation of the ground side (This is a guess)