<label for="phone">Enter your phone number:</label>

<input type="tel" id="phone" name="phone"
       pattern="[0-9]{3}-[0-9]{3}-[0-9]{4}"
       required>

<span class="note">Format: 123-456-7890</span>

label {
    display: block;
    font: 1rem 'Fira Sans', sans-serif;
}

input,
label {
    margin: .4rem 0;
}

.note {
    font-size: .8em;
}