Creates a contact.

Passing a phone number that is already used by an existing contact would get and update the existing contact. It would not try to create a new contact with a duplicate phone number.

Passing an external ID that is already used by an existing contact would get and update the existing contact. If the parameters include a valid phone number and email that differ from those currently associated with the contact, the existing values will be updated accordingly, as long as they don't conflict with other contacts' values.