If that is the case @Anne1438 , all the guest has to do is call Airbnb and explain the problem and ask for Airbnb's help in resolving the issue.
Has the guest done this yet (presume that they have as it is two days since you first posted)?
Either there is a problem with the guest ie there is something wrong with the payment method or it is a fraudlent booking (in which case you don't want them) or there is some sort of technical error. Whatever is wrong it can only be resolved by your guest calling Airbnb. There is nothing wrong with your listing. I have just gone through to try and make a booking for tonight and it takes me through to making a payment.
Exchanging numbers wouldn't help you in this situation as it wouldn't resolve the problem. Airbnb makes it clear you can't share contact information before a booking is made both for your safety and the guests and also as it stops hosts and guests trying to make private arrangements outside of Airbnb.