number

The phone rings:

[Mum's croaky scared voice] "I've got a bit of a problem"
"What's that then?"
"I want to tell someone my phone number... but I... can you tell me what my phone number is?"
"Okay, have you got a pen there?"
"Just a moment... okay"
"Right, it's.... [I slowly list the numbers of her phone number]"
"Okay, now let's see what I've got in this book"
[looks in her address book]
"I've got ..... [she reads out MY phone number]"
"Mum, that's MY number"
"Oh" [stunned]
"Did someone ask you for my number?"
"No"
"Well, you've just read out my number"
[exasperated] "Well, I want to GIVE people your number"
"No, Mum, you've just rung me to find out your own number. You've just written it down"
[incomprehension]
"Mum, who did you want to tell your number to?"
"Sheila and Don. They ring me all the time, you know"
"Why don't they have your number already, then?"
"Because it's changed, hasn't it?"

[this is the point I realise that Mum thinks her number has changed because I've replaced her telephone with a simpler model]