<input type="tel" placeholder="Enter Mobile no" onkeydown="return validateNumber(event)" required>

function validateNumber(event) {
  var keyCode = event.keyCode;
  var excludedKeys = [8, 37, 39, 46];

  if (!((keyCode >= 48 && keyCode <= 57) ||
      (keyCode >= 96 && keyCode <= 105) ||
      (excludedKeys.includes(keyCode)))) {
    console.log("alphabets are not allowed");
    event.preventDefault();

  }

}