Over the years, I have designed many disaster recovery plans when deploying a new business phone systems.
What do they say about the best laid plans?
You can have multiple carriers, automated dial tone fail over, and the most carefully designed plan... and then a dump truck goes down the road with the bed up and...snap...the lines to your building are gone and you are out of service (this happened).
Or, you build a hot Disaster Recovery site with redundant services and live sync between the sites and two teenagers build a camp fire on the eastern bank of the Housatonic River. They melt a major fiber optic cable and take down much of the Northeast (this happened also).
Can someone say "self healing Sonic Ring"? Increasingly complex networks have increasingly complex problems. There are no 100% redundant solutions. What can go wrong, will go wrong.
Here's an easy, inexpensive and quite reliable option to make sure your customers can reach someone if your regular telephone services are down, your office is unaccesible, or there are campers in your neighborhood:
1) Make sure you have call fowarding as part of your telephone services package. This is usually free. Some carriers offer direct trunk overflow (DTO) which will automatically failover to another phone number. Make sure you know how to activate this service and have the number to call to activate this service handy.
2) Set up a cell phone account. This can be someones personal account, or better, an account set aside for emergency purposes. You don't need to have a ton of minutes or an expensive phone. We are really just interested in the cellular phone number and plan.
3) Make sure the cell account has a voice mail box. Record a greeting that tells callers if they have reached this message, there has been a disruption in the business phone service and ask the caller to leave a message. Tell them someone will return your call shortly.
4) Activate the voice mail to e-mail feature of the cell providers voice mailbox and have the e-mail delivered to a e-mail address that will be accessed by key people. This should be a g-mail account or some other cloud based e-mail provider.
When your service goes down, call foward your number to the emergency cell phone number and then log onto the e-mails account to retrieve the voice messages. You can then either call the folks back, or forward the messages to others in your company to handle.
This works as long as your regular carrier is not so disabled that they can't forward calls and the cell provider is not also out of service. For most incidents, this plan works well.