This question type is used for situations when you are looking for a numerical answer (e.g. rating on a service, amount of money in the bank, etc.)
The answer to the number question is stored as number data type.
For phone numbers, you can use phone content type.
Limiting answers to a certain range
You can force the responder to pick numbers within a desired range by typing in enabling Minimum value? or Maximium value? options and typing in your upper or lower bound. This way you can ensure that your responder is not typing in negative numbers when they should not be or giving you a 11 out of 10 for the services you have rendered.
Formatting the input field for a number question can help the responder anticipate what format the answer should be in.
By clicking Is money? you can add a dollar sign in front of the input field to let the responder know that they are expected to put in a dollar amount. Alternatively, you can put in text in the Suffix field which will show up to the right of the input field.
Changing the input style to slider will enable the responder to submit answers by dragging a slider.
You can add labels to both ends of the slider to inform the responder about what the number answer would imply.