Issue #2970912 by zaporylie, Chi: Missing configuration schema for default value of Telephone field type

core/modules/telephone/config/schema/telephone.schema.yml

@@ -15,3 +15,11 @@ field.widget.settings.telephone_default:
     placeholder:
       type: label
       label: 'Placeholder'
+
+field.value.telephone:
+  type: mapping
+  label: 'Default value'
+  mapping:
+    value:
+      type: string
+      label: 'Value'

core/modules/telephone/tests/src/Kernel/TelephoneItemTest.php

@@ -36,6 +36,7 @@ class TelephoneItemTest extends FieldKernelTestBase {
       'entity_type' => 'entity_test',
       'field_name' => 'field_test',
       'bundle' => 'entity_test',
+      'default_value' => [0 => ['value' => '+012345678']],
     ])->save();
   }