I've had the dubious pleasure of sharing a liveaboard with a couple who had two of those.
Every 1-2 minutes they would sound off to call their beloved over to show their latest discovery.
Here a honk.
There a honk.
Everywhere a honk-honk.
Did I mention they were using them every 1-2 minutes?
Honk, honk, honk, honk, honk, honkity-honk-honk-honk...
It was quite possibly the most annoying device I have come across in diving. They were completely unware of just how much the other divers on the boat came to despise them for their noise-making antics.
So be warned that there are dangers to the overuse of noise-making devices underwater. For only emergency calls, like your have planned, they work ok- they do make noise.
Personally I've never found a real need to make noise underwater. I find agreeing, on the surface, to maintain contact and dive as a buddy team works just as well: agreeing to glance at your buddy and your surrounding every 30-60 seconds or so.
Cheers,
Rohan.