document phone form field

Author: mjauvin

backend/forms.md

@@ -613,6 +613,34 @@ user_password:
     type: password
 ```
 
+### Phone
+
+`phone` – renders a single-line input for phone values with built-in browser validation.
+
+```yaml
+telephone:
+    label: Phone number
+    type: phone
+    pattern: "[0-9]{3}[0-9]{3}[0-9]{4}"
+    placeholder: xxx-xxx-xxxx
+    maxlength: 20
+    minlength: 12
+    size: 20
+    required: true
+    options:
+        514-123-4567: First Test Phone Number
+        800-111-2222: Second Test Phone Number
+```
+
+- Displays a phone icon on the left in both edit and preview modes.
+- In preview mode, renders as a clickable link that opens a phone dialing screen on mobile.
+- Supports all standard HTML5 attributes for `<input type="tel">`:
+    - `placeholder`, `maxlength`, `minlength`, `pattern`, `size`, `list`, `autocomplete`, `required`, `readonly`, `disabled`
+- `options` will be rendered in a `<datalist>` element, enabling autocomplete suggestions.
+- If an option's value and label are identical, the label is omitted for brevity.
+
+See [Defining field options](#defining-field-options) for the different methods to specify the options.
+
 ### Radio List
 
 `radio` - renders a list of radio options, where only one item can be selected at a time.