Job: To alert their deaf or Deaf handler to specifically trained environmental sounds, including, but not limited to, alarms, doorbells, knocking, phones, cars or their name.
Handler: Likely won’t show signs of disability. May or may not speak verbal English.
Gear: Hearing Dogs don’t require special gear, but many state laws designate bright orange as reserved for Hearing Dogs.
Notes: Hearing Dogs can be trained to respond to any environmental sound or cue their handler needs to know about. Just because you can’t see what a Hearing Dog is responding to doesn’t mean he’s not working.
excerpt taken from http://www.anythingpawsable.com/types-of-service-dogs