I received several enquiries from guests asking to call them back on their phone number written in a funny way like "34plus56plus32plus45" instead of 3456 3245.
For security reasons airbnb block out personal info like phone numbers in the message thread. Personal info is only revealed after a booking is confirmed.
Not responding can affect your response rate, so you should simply decline or send a special offer for a ginormous amount.
They are trying to get around AirBnB policy of using the platform to make bookings and communicate. Do not fall into the trap of communicating with these guests offline - at worst it will be a scam, at best they want to give you cash and not pay the AirBnB booking fee (and of course if you take cash you are not protected in anyway by AirBnB)