The phone number field has 2 supported attributes: country and number. To pre-populate these attributes, you can use the field code along with the attribute.
phone.country (It supports only iso2 country codes)
phone.number
If you miss either parameter, the phone number response will not be pre-filled. The field code can be used in URL parameters, as shown below.
https://example.neetocal.com/meeting-with-oliver-smith?phone.country=GB&phone.number=2071234567