kosher alarm clock: what's kosher about it?

I’ve been seeing ads for quite a while marketing kosher alarm clocks do be used on shabbas. They have special features which will minimize chillul shabbas. However there’s an issur derabanan called avsha milsa. Which is an issur forbiding people from doing something before shabbas which will cause a reaction on shabbas that makes a noise. People might think that you did that melacha on shabbas. R’ moshe feinstein writes that alarmm clocks would fall under this issur if they can be heard out side the bedroom by other occupants of the house because normally an alarm clock is set the previous night before going to sleep. I am astonished that there would be a company that markets kosher alarm clocks. I believe that an alarm clock on ones cellphone would not fall under this issur because once you input a time you don’t have to turn it on every night.