I posted a problem about WiFi calling on Lebara UK in the P5 section of the forum, I've since had a reply from Lebara stating that WiFi calling will only work on the P6a and later. So far as I can tell, this is because the necessary updates to allow WiFi calling were pushed out by Google after the final updates were made for the P5.
So, I'm wondering if whatever was pushed out by Google to allow WiFi calling on Lebara and other MVNO's for the P5 could be included in a future update for the P5? I know that the P5 is now a legacy device, but it still works just fine.