Well. you have to use your brain. The iPhone will play your ringtone until the voicemail picks up. But, if youre not using the phone, the ringtone will play out. So, knowing that, you make a 2:50 min ringtone instead of the normal 25 seconds and when you make that particular ringtone your alarm, it will play out. Job done.
Haha I do the exact same thing. Put it right under my pillow so it's a lot easier to turn off. Actually, for that reason I set 4 alarms all to go off within 5 minutes of each other. Darn my habits for wanting some quality sleep.