How do flashing antennas on cellphones work?

How do flashing antenna's for cell phones work? Can you use them on retractable antenna's. Are they worth buying?

Hi,

I bought one a couple years ago when it was a new thing. It worked by screwing off my antenna top (that didn't retract or move at all), pushing in 2 batteries, and then screwing on the clear antenna that flashed blue when someone called.

It really only worked for a couple months, and finally died. Plus, it was hard to find the tiny batteries.

It looks like you can buy them for retractable antennas on eBay (a quick search showed me a few results), but it depends mostly on your phone model.