Old Account Migrator addon for Digits has an option to as for mobile Phone Number as compulsory or optional just after login or signup.

To set this up, you just need to install the free addon and set it up

1. Set Show “Add Phone Number” Popup to Old Users to Yes
2. Set Make it compulsory to Yes if you don’t want your users to access the site until they enter their Mobile Phone Number
3. And lastly set When to Show setting according to your needs.

Once done, all your users who signup or login using Social Network will be asked to fill up their mobile phone number in their account.