Creates a user phone number

curl --request POST \
  --url https://api.rootly.com/v1/users/{user_id}/phone_numbers \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/vnd.api+json' \
  --data '{
  "data": {
    "type": "user_phone_numbers",
    "attributes": {
      "phone": "<string>"
    }
  }
}'

{
  "data": {
    "id": "c56ea00d-a848-4f15-bc92-09b960ef0880",
    "type": "user_phone_numbers",
    "attributes": {
      "user_id": 513,
      "phone": "+14155552671",
      "primary": true,
      "created_at": "2025-10-01T19:31:48.882-07:00",
      "updated_at": "2025-10-01T19:31:48.882-07:00"
    }
  }
}