This endpoint will create a contact, or update it if the phone is already in the database.

All your specific fields - called customFields in Simio - can be assigned to this contact through the JSON customProperties.
Note that customFields are always lowercase, without whitespaces.
Note that customProperties are always Strings.

For instance:
customProperties: {
discount_code: "lSHAJB68",
cat_name: "Lynx"
}