I have enabled phone verification to "always" for online bookings but my customers did not receive a code, why is that?

They need to use the phone number with the country code. If you don’t add the country code, the system doesn’t have any way to identify the country and can’t send the SMS. Sometimes it will fill out the country code automatically, but not always. In most cases you replace the initial 0 of the phone number for the country code.