....we had a Casio, with a 30 minute alarm velcroed into our helmets to do the same thing. I'm now trying to recall what happened if you didn't launch on the hour or half hour. Maybe we adjusted it in the brief, I really can't remember. British ingenuity (cheapskates) at its best again....
We had the same thing in Bahrain and KKIA. A cheapo Casio from the souk with a stretchy rubber strap which could be adjusted so that the back of the watch pressed against the headset earpiece - the half-hourly beeping was just the right volume level.
The switchology to turn the things off was a bit complex - press this, twiddle that etc., so most people just left them to keep beeping until a battery change was needed. Although a watch change was simpler. If you walked past the aircrew clothing area, it was like a Star Trek tribble nest every half hour as all the watched started peeping away in unison!
But, like the mythical Russian space pen, the little Casios did the trick very well indeed.