Check out example codes for "html phone number input". It will help you in understanding the concepts better.

Code Example 1

The <input type="tel"> defines a field for entering a telephone number. Note: Browsers that do not support "tel" fall back to being a standard "text" input. Tip: Always add the <label> tag for best accessibility practices!

Define a field for entering a telephone number:

<label for="phone">Enter your phone number:</label>
<input type="tel" id="phone" name="phone" pattern="[0-9]{3}-[0-9]{2}-[0-9]{3}">

Code Example 2

JS Libraries required :
======================
https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js
https://cdnjs.cloudflare.com/ajax/libs/intl-tel-input/8.4.7/js/intlTelInput.js

Inside JS File :
================
$("input").intlTelInput({
  utilsScript: "https://cdnjs.cloudflare.com/ajax/libs/intl-tel-input/8.4.6/js/utils.js"
});

Inside HTML File :
===================
<h1>International Telephone Input - BOOTSTRAP INPUT GROUP</h1>
<form>
  <div class="input-group">
    <input type="tel" class="form-control">
    <span class="input-group-addon">Tel</span>
  </div>
  <br>
  <div class="input-group">
    <input type="tel" class="form-control">
    <span class="input-group-addon">Tel</span>
  </div>
</form>

Inside Css File :
=================
body {
  margin: 20px;
}
form {
  width: 300px;
}

// workaround
.intl-tel-input {
  display: table-cell;
}
.intl-tel-input .selected-flag {
  z-index: 4;
}
.intl-tel-input .country-list {
  z-index: 5;
}
.input-group .intl-tel-input .form-control {
  border-top-left-radius: 4px;
  border-top-right-radius: 0;
  border-bottom-left-radius: 4px;
  border-bottom-right-radius: 0;
}

Code Example 3

<label for="phone">Enter your phone number:</label>
<input type="tel" 
  id="phone" name="phone" 
  pattern="[0-9]{3}-[0-9]{2}-[0-9]{3}">

Code Example 4

<div class="input1">
      <input type="tel" placeholder="telephone">
      <p class="info">Telephone must be a valid US number (11 digits)</p>
    </div>

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