hearing dog 
noun
A dog trained to alert people who are deaf or hard of hearing to such sounds as the ringing of an alarm, doorbell, or telephone.Yet it costs just as much to train a hearing dog and several months of training....
- Matthew led a sponsored cycle ride to help to buy a hearing dog for a profoundly deaf teacher at the college.
- If you start with a young puppy, the amount of time isn't usually too much more than what would be needed to train a hearing dog: about 30 to 45 minutes a day, plus one hour a week when you are in an obedience class.