Business Voice "System Unavailable"

Recurring problem..... Customer calls us they get a message system unavailable...... customer thinks we're out of business... EQUALS loss of business. I have called on this numerous times. Still happening. Need to speak to someone high up in the company to resolve this issue. Comcast says give me numbers of who called with date and time, maybe a specific carrier problem. Info was furnished, still happening. It even occurs when we try to retrieve messages on our own phone. I pay every month for a service and service is not only lacking but is causing loss of revenue for my business. NEED THIS RESOLVED NOW!!!