To get any additional properties returned, you will need to add a query parameter to your call. Specifically for phone number, the call would look something like this
https://api.hubapi.com/crm/v3/objects/contacts/#id?properties=phone&hapikey=xxxxx
You can get a list of all available properties using the properties endpoint or looking in the settings of your portal. Make sure to use the property internal name.
You can also test your API call structure using the API docs. Check out this screenshot as an example
