Brian Gregory wrote
Peter Able wrote

Surely the only rule is that it is discourteous to
confirm back a number NOT in the format first
spoken. E.g. 23 93 73 confirmed back as 239 373 !

People do that to me so often!

Me too, because they are reading the number
back in the format its displayed on their screen.

And then usually I genuinely cannot
say whether they have it correct or not.

I always can, tho I have to pause to
think about it with my mobile number.