Are they legally required to ask for your number or do they want it for themselves?
Are there any free online numbers that will work?
There are two reasons:
1. They do this to verify who withdraws money
2. There is a law from the government that pushes them to verify you.
In any way, they have the right to ask you for this kind of KYC, especially when it comes to bitcoin wallet where they want to be sure that their service isn't for money launderers and in case something happens, they'll be able to provide all the necessary information from their side that's required by the government.
If you have withdrawn money via the contactless method (not by inserting your card in the ATM), you would be asked to enter the message from your phone (It's just an example of one case from personal experience).
So, it's pretty logical and if everything is okay, I don't see any problem in it.
If people want to money launder they could either buy a burner phone with cash, or just buy bitcoin, convert it to monero then send it somewhere to convert back to bitcoin, or send through a mixer and it's as clean as can be.
