You can script user creation with Salesforce DX. The force:user command tree has what you need:
force:user:create creates a user, given a user definition file. It takes optional flags generatepassword=true and permsets=<Name> to configure the user immediately. The user definition file lets you set the profile and other user attributes.
I don't know a way to do that, but here is what could be a workaround (not tested, please tell me if it works !)
Create a file named setUsersMobile.apex
String phoneNumber = '+33 6123456785'; // Set your real number here maybe
List<User> userList = [SELECT Id, Name,MobilePhone FROM User WHERE MobilePhone = null LIMIT 1000];
for(integer i=0; i<...