Hold button for UK Landline telephone

Question

We have retro rotary telephones, and I'd like to make a hold button for the one in my office in the UK. This is literally a "ye olde rotary phone" (1940s) made from bakelite

I have fairly basic electronics skills but I don't actually know how the phone works, in that sense anyway.

Is this just a case of switching out the circuit to one playing "The girl from Ipenema" or is it more complicated than that?

For example, what is it that keeps the line open if I make an outgoing call? Could I accidentally cut the call or does the line need to go open circuit to do that? What are the voltage limits?

(I would just buy one, but can't find any!)

[UPDATE]

Some good points have been made in the comments that bear highlighting.

When I used to work in a shop repairing computers (about 40 years ago) we also sold telephone handsets to the public; the ones with green stickers were allowed to be connected to the phone network, the ones without - or with red stickers - were not. I'm pretty sure the same or similar system exists today, especially with digital exchanges, "digital voice" etc.

So, to be clear, nobody should be connecting unauthorised equipment of any kind to the telephone network.

Answer 1

If these are phones that work the same as genuine ye olde rotary dial phones, then, a telephone line will be "held" by placing a resistor across the line that draws at least the minimum required by the system. This will vary by system but I'd guess that a 1k resistor would work. This can be determined experimentally or by asking the system supplier or finding a specification (which probably does exist for most systems).

To place a call on hold (other than leaving the phone 'off hook', "The girl from Ipenema" needs a suitable resistance across the line, and switching must becarried out "make before break".

To make an outgoing call "The girl from Ipenema" must be first removed

Answer 2

Telephones are two wire appliances that are grounded only at the exchange end.

A hold-music circuit could go something like this:

^{simulate this circuit – Schematic created using CircuitLab}

The resistor will probably get hot so use a 2W part (or 4 half watt 150 ohm resistors in series etc.)

600 ohms is a guess, measure the current and voltage during a call to get a more precise value.

"Music source" should be at approximately headphone power level.

This is not the best solution it is just the simplest.

There are laws about connecting equipment to the telephone network. Sometimes these are protectionism to stifle competition and innovation, but usually they are there to stop people from connecting dangerous equipment to the network that could harm phone company equipment and employees. If you connect something that behaves very much like a telephone and is safe no-one will care.