Once a booking has been confirmed, the host then has the guests phone number, given to them by AIrbnb, so they can contact them outside of the platform. I often contact guests by text or even call them if there is an emergency or if I want to let them know I've saved a parking space for them and they should call when outside the house so I can move my car. I do believe it's the same for the guest but I may be wrong.
The policy break is if either the host or guest try to exchange phone numbers before the booking is confirmed.
Sounds like your booking was already confirmed so the host would have been given your number by Airbnb. Why would they give out phone numbers if you're not supposed to contact each other??