Having a valid and verified phone number from all your guests is very important.
Requiring a phone number is probably something you want to have for all your guests, irrespective of the listing they select.
On the Custom Codes tab, select All listings to make your custom codes available without restriction to a particular listing.
You can just create your custom code (for example %%no_phone%%) with your own message, such as "Please do not forget to add your phone number to your Airbnb profile, it you be very appreciated in case of an emergency.".
Then, set the condition to "The guest has no phone number". Only users who have not verified their phone number with Airbnb will see this message.
You will only have to include the short codes %%no_phone%% in the templates you want.