Dial out over IAX and Analogue

My Dialplan has calls being routed over IAX (to Voiptalk) if they are a UK trunk call, and over Analogue when it is a local call.

I want to be able to handle a dead ADSL line, so that it routed all calls over the analogue circuit. Is there a feature in the Dial application to allow me to handle a dead network connection? Also does the dial application fall through when the outgoing IAX line is busy?

I am not quite clear as to the Dial apps behaviour.