It is difficult to provide trunk guidance for resilience as the configuration of the external trunk routing and usage of every network varies greatly. We can only discuss general principles and factors that need to be considered, and show examples of the different methods that can be used.

• Special consideration must always be given to ensure the correct routing of emergency calls, especially in regard to identifying caller location.

• Outgoing caller ID must be assessed. Rerouted calls may be blocked if the ID used to call out on an alternate trunk or site is not accepted by the line provider.