I'll have to check when flying if they list what they block before you pay. I would think they would need to, but probably not.
The most likely candidate to work would be Signal, but I think that only handles calls between people using that app. It's all encrypted, so unless they block the app entirely, it should be good.
Unless they block ssh, you could use any VoIP with a tunnel, but that's not something the common person is going to be able to set up.