Currently there is default soft validation on the mobile number field for customer. This new setting will piggy back on that mobile number formatting, and now not allow for the record to be saved if the formatting of the mobile number is incorrect or unrecognised. This setting does not enforce a mobile number.

Settings > Company Settings > My Company - Mandatory Fields

Note: This does not mandate for the mobile number to be entered, but it will only accept a value in this field that is a recognised format.