Having trouble resetting phone.

So my mother has asked me to reset her old phone, she doesn’t remember her Apple ID password. So I’ve pressed ‘don’t remember password’ as you do, and then it asks for a trusted phone number, so then I put my mother’s phone number (it’s the only trusted phone number she has), and a window pops up saying ‘Verification Failed - There was a problem connecting to the server’.

How do I fix/solve this?

Please and thank you.