Took me a moment to get this, but the part I still don't understand is why he needed to encode the phone number like that?

Does Airbnb strip contact details out of messages to prevent hosts and visitors from getting in contact directly?

Oh yes. They want all contact to go through them. They charge huge fees and they want to make sure that the hosts and clients don't try to make arrangements on their own without AirBnb getting their cut.