input: Write a python function to check if a given phone number is valid.
  assert check_phone_number('8888881111') == True
  assert check_phone_number('12345') == False
  assert check_phone_number('12345678901') == False
  assert check_phone_number('123456789a') == False
  assert check_phone_number('0987654321') == True
output: def check_phone_number(phone_number):
      if len(phone_number) != 10:
          return False
      if not phone_number.isdigit():
          return False
      return True