I lost my iphone but the alarm is still on and I want to know if the alarm turns itself off or if someone calls the iPhone will the alarm turn off.
The alarm will last about 15 minutes, according to Anna Fyen's answer on Quora:
My alarm works fine every weekday. I cannot give you the exact time. But one day, I set an alarm and let it ring for a while so after about 15 minutes or so of ringing it turns itself off and displays a message on the lock screen.
Phoning an iPhone currently playing the alarm will likely interrupt the alarm and replace the sound with that of the ring tone.