Check out IQ Services out of Minneapolis, Minnesota provides a 100% service model with no hardware or software to purchase or the need to train your employees. This service comes with 24/7 notifications and a online portal where you can listen to all monitoring and notification calls that have been placed.
The best thing, this is done remotely like a real customer calling in taking into account outside influences you cannot test from the inside only.
1 to hundreds of calls are placed that exercise the system as many times per hour that is needed, and each call is being checked for and reported back on call quality, availability, times at each step, proper greeting and what the customer feels is important to know.
They offer a trial as well if interested. http://www.iq-services.com/heartbeat.asp