HTML form validation using pattern error (please match requested format)

<form>
    <p>
        Telephone:
        <input type="tel" name="telephone" required="required" pattern="[0-9]{3}-[0-9]{3}-[0-9]{4}" placeholder="122-345-6789" />
    </p>
    <input type="submit" value="Submit" />
</form>

I'm not sure where this error is stemming from.