Why can’t I call some of my contacts in Skype for iOS?
This may be because you are trying to call a contact from the main address book on your iPhone or iPad, rather than from your Skype contact list, so make sure you are viewing your contacts within the Skype application.
If a contact from your Skype contact list doesn’t have a Skype account, they cannot participate in a video call. You may have added a new contact by manually typing in a new name and phone number in your Skype contact list, but if no Skype account is associated with that person, you can’t video call them.
However, you can still use Skype to make low cost calls to that person or send them a text message. You'll only need a little Skype Credit or a subscription. Simply tap Voice Call or SMS.