More and more websites nowadays require new users to verify their 'identity' by requesting the user prove that he/she has access to a device identified by a telephone number (typically a phone).
The often cited reason for this is the rather non-specific: "to protect your account".
I agree that, in case the user is otherwise exclusively using desktop/laptop computers to interact with the service, using a phone as an out-of-band verification of 'something you have' could effectively been seen as two-factor authentication.
However... over 50% of all online access is now done using mobile devices, such as smartphones.
Does the use of a mobile device, to prove that the user has access to said device, undermine the implied added security?