On your booking form, you can collect all the information you need from your client, before you meet.
There are a variety of questions that can be added under Booking form Questions.
How to add a phone number question to your booking form
When you add the question type Phone Number to your booking form, YouCanBook.me will automatically detect the location of the people booking with you so there isn't any special formatting required.
How the booker enters their phone number
When the booker sees the Phone Number question on the booking form, it will auto detect their location and country code based on their IP address. If that is not correct for their location, they can change it in the dropdown menu.
The flag takes care of the country code for the booker. They just need to enter the phone number as they normally would. No special formatting is required.