You can keep a phone number and email address in your Contact Info, but you must have a Preferred Contact Method. This means you can only receive one type of notification: email, SMS, or Slack.
To set or change your Preferred Contact Method, or another Staff Member's Preferred Contact Method, go to "People." Click on your name or the name of the person whose preference you wish to set.
Look for "Contact Preferences" on the lower right of the Person Profile:
Click the dropdown and choose the person's Preferred Contact Method. Hit the "save" button: