If you business depends on in-coming calls to telephones for its survival you need to consider the BCP aspects of your telephone system. If there is a major failure in your telephony switch (PABX) how will your customers call you? Are there alternate numbers they can call or does your telephony system allow your phone numbers to be immediately diverted to an alternate switch in the event of an outage?
What happens if the cable from your local exchange becomes damaged?
BT offer solutions to these type of issues such as Dual Parenting, Dual Routing, and Route Separacy. There will be a set up and operational cost associated with these services and they may not be offered in all locations. Similar services can be provided by third party companies such as Six Degrees Live Number Service and Gematech's DDI Call Recovery service.
If your business needs call recording as part of its business process you will need to give further consideration as to how this can be achieved in a outage event. Smaller businesses may wish to consider a hosted pabx service where these issues are handled by the provider.