As was already mentioned, the 7230 and 7290... in fact, the entire 7200 series... have INTERNAL antennas. So, there's no way to attach a flashing antenna.
HOWEVER... they DO have a small, red, LED in the upper right corner. You can set it to blink when you get new messages. You can set how many times it blinks. You can even set it to blink with incoming phone calls. In your sound profiles, you turn the LED usage on & off from a setting called Repeat Notification. That should at least let you get close to what you want it to do.
Hope that helps.