If I remember correctly the setting is in the advanced External Line properties, and it's called something like "Chain to next line." Basically it lets you select another External Line to send calls to if the current one fails to place the call correctly. So if you have 4 lines, you want to chain 1 to 2, 2 to 3, 3 to 4, and 4 to 1 to ensure the call will get through just so long as any of the lines are free.