Sleeping thru ringtones

The company I work for requires that our technicians be on call for over night shifts. unfortunately, several of the guys are heavy sleepers, and have slept thru the ringtones and have gotten in trouble at work (or had their spouse woken up by the phone and gotten in worse trouble at home)

I was wondering if there was an accessory out there that might help - something they could wear while sleeping that would vibrate when the phone rings (If I had a clue how to build things, I would make a small vibrating attachment for a watchband so they would feel ir vibrate, but their spouse would not hear it ring.)

Any suggestions would be appreciated!