I had some Skype credit which allows me a few more calls on Teams Dial Pad, and Teams can work on a mobile phone via their App. I also have call out options with my work Teams but that's not a public proposition. Viber is probably the biggest next player in this market. You may be able to get a UK based VPN to permit wifi calling in non UK scenarios.

In the medium term I would expect WhatsApp calling to be enabled more on BA, as it is on many (most?) of BA's rival airlines. It is already used in some limited circumstances, notably BAH and irrops.