Hi, it is against Airbnb policy to exchange personal information including contact numbers prior to a confirmed booking as such if you send messages containing such information, Airbnb block those details. This is for the safety of both the guest and the host also if you have any issues down the line, you might loss the backing of Airbnb if you have breached their policy.
I don't necessarily think it is a fraud, some people prefer to communicate by phone and he may just not be aware of the policy or then again, he might be but trying to get around it, who know but best to play safe.
I'm assuming he has sent you a reservation request or enquiry. I would respond thanking home for his interest and informing him that Airbnb only release personal information once a booking is confirmed. However pending a confirmation, you are happy to answer any questions he has via the Airbnb platform and ask if there is anything in particular he would like to know.
It is important to note that should you or the guest have any issues down the line and required assistance from Airbnb, that Airbnb only accept communication seen on their platform, they do not accept text messages, WhatsApp, personal e-mail etc. as evidence of communication.
All the best,
Ama