Upgrade to a "bat signal" instead of a bell
Your special BatPhone doesn't have to sound like an ordinary phone. Instead of a regular ring it can be equipped with an exciting attention-getting bat-like electronic ring signal. It's powerful enough to wake up Alfred. This is NOT the same as the sound effect on the TV show, but is exciting, unique and attention-getting.
The $24 price is in addition to the price of the phone, not for an electronic signal purchased alone. The bell will be removed when the electronic signal is installed. The signal is pre-set to a "medium" volume level, but it can be easily adjusted.