Lets say we have 2 BRI interfaces (BRI0 and BRI1) and the service-provider has configered them for different ranges of telephone-numbers.
Lets say we have 10 Phones. I want the first 5 phones to use BRI0 exlusively and the latter shell use BRI1 exclusively. Both BRI interfaces are in the same router. Spreading interfaces on two routers is therefore not an option.