Default value in input function (HTML) that can't be removed

So I'm trying to create a form where user will enter their phone number. Since it's supposed to be used in single country, I want country code as a default value that can't be erased, only added to. How that could be done, or maybe there are alternative best practices for such case?

<form class="form__wrapper">
            <h2 class="form__wrapper_text">Just form</h2>
            <div class="form__wrapper_number">
                <p>Your number:</p>
                <!--<span>+380</span>-->
                <input type="tel" value="+380" o>
            </div>
</form>