There are a few methods to allow a number, using the CallDetector app for Android.

You can access the Allow List view on the Allow tab and manually enter a valid 10-digit number, then tap the Allow button.  A pop-up message will follow, confirming that you would like to add the number to the Allow List.

OR

You can go to the Activity tab, tap on the number you'd like to block, and select the Allow button on the detail view.