How do I declare a Telephone field in a content entity?
Okay, the following configuration seemed to work:

$fields['display_number'] = BaseFieldDefinition::create('telephone')
      -&gt;setLabel(t('Display Number'))
      -&gt;setDescription((t('The phone number to display')))
      -&gt;setDefaultValue('')
      -&gt;setDisplayOptions('form', array(
        'type' =&gt; 'telephone_default',
        'weight' =&gt; 0,
      ))
      -&gt;setDisplayConfigurable('form', TRUE);