Force user enter phone number with only digits.

this.$phone_number = $(this.$elements.phone_number);
this.$phone_number.on('input', this.myHandler);

private myHandler = (): void => {
  const phone_number = this.$phone_number.val() as string;
  const digitRegex = new RegExp('^[0-9]*$');
  const validPhoneNumber = digitRegex.test(phone_number);

  if (phone_number.length > 0 && !validPhoneNumber) {
    this.$phone_number.val(phone_number.slice(0, -1));
  } else {
    this.$phone_number.val(phone_number);
  }
}