You can also leave your number of your wife to the guests in the message Airbnb after the confirmation of reservation and tell them it's for Thais speaking.
I think you can't add a phone number to your listing at all, as it would be against the Airbnb rules. If you did, potential guests could contact you or your wife directly without booking, which would circum-navigate the system. When guests book they get the phone number anyway - here is where to add the number for contacts and arrangements. I stand to be corrected! Hope this helps. Happy hosting.
You can just go to the profile and add a phone number.
Phone Numbers
This is so your hosts, guests, and Airbnb can contact you. This is the number for guest contacts, booking requests, reminders, and other notifications.
You can also leave your number of your wife to the guests in the message Airbnb after the confirmation of reservation and tell them it's for Thais speaking.