On Windows 10 Mobile, we ask you to verify your phone number if you want to automatically add people from your address book who are also on Skype.
Skype will use this information to verify your contact details and allow others to find you on Skype. By verifying your contact details, we can help make sure that we're adding the right people when automatically adding friends.
To change the setting later, open the Skype Video app, and then select More > Settings > My phone numbers under Manage how people find me on Skype.
For more info, see Why we check your phone for contacts.