Is the in-app purchase to upgrade to Premium a one off payment?

No, the Premium upgrade is a yearly subscription, which gives you unlimited access to all unlocked features, including storage for your call recordings, settings and purchases.

The subscription automatically renews every year unless you turn off auto-renewal in your Apple ID/Google Play account settings.

You can use the app and the unlocked features for Christmas, and even during the rest of the year, as long as the app exists and your subscription is active.