In DJ-Classifieds, you can limit the access to contact details to registered users only. Let's see how to do it.
Go to DJ-Classifieds component -> Options -> "Global" tab.
Search for setting "Contact only for registered" and set it to "Yes".
Let's see how it looks like on the live page when you are a non-registered user. Now the contact information at Ad details should not be visible.
If you click "Please login to see contact", the link will direct you to the login page, and after successful login, you will be redirected back automatically to this ad.
And now see how it looks like when you are a registered user. Remember that you must be logged in to see the contact details. Now, as you can see on the picture below, contact information in Ad details is visible.