is it true

Only if you're using the same make and model of phone, have the same tariff with the same provider and can access the phone you're calling from the same relay mast as you're using.

So technically the answer to your question is "yes".

no

maybe

it depends

Depends on how far the signal has to travel,

If they're close by, you can go and whisper in their ear ... but only if yours is the same model ear, I mean phone.

And they can mouth back, so as not to be overheard "Fook Off, you daft tw@t".

If you have two baked bean cans, and a long piece of waxed string, will it work if one is Heinz and one is Branston?

Also, if you have supermarkets own "Saver" beans will it have the same volume?

AP you've shocked me.
I never thought I'd see your name and Supermarkets Saver Beans in the one post.

Sorry, I dropped my guard, so now the truth will out!

Is it safe?

It is but only if its the same number as yours.