If you need an overview of the leads on a specific campaign, and you have the option to do so as follows:
1. Select "Dialer" from the menu on the left.
2. Next, click on the three dots to the right of that campaign and then "Lead Overview":
3. You will now see the following image:
There you can see how many leads you can call (Open topics), how many are on the campaign but are locked by phoners (Locked leads), and how many have been closed (Closed leads).