I've noticed a few issues with orders when a user enters a landline number in the box which says Email or mobile phone number on the checkout screen. Yes it states mobile number but some customers enter landlines.
Unfortunately, I believe that an SMS is attempted to the landline. Depending on the landline this could either be spoken to them or not sent at all. The first is annoying, the second prevents someone who is responding to a abandoned cart recovery email from placing their order because it sends a verification number to the landline, which they obviously can't use.
We believe we have lost multiple orders through this, while I understand it does state mobile phone on the text box many customers still enter landlines.