One more tip:
If you insert digits in your route-pattern for an alternate carrier, make an extra entry below it wich is the same as your original route (so without the extra digit insert).
This way, if the alternate carrier has problems, calls will still go out using the original carrier. If you inserted the digits through ARS, you can do the same, but then you would delete those digits on the second entry in the RP.
Pay attention to CDR, because you will not notice when the alternate carrier fails (because the calls still get through, and there's no angry people complaining). But most of the time it costs more when your not able to make a call, than when you pay a few cents extra, plus the fact that errors tend to occur when you're not in the office, but on the beach or something.
Oh, one more thing (maybe obvious, but very important), make sure the emergency number is not affected by this! Route the emergency number to a seperate route-pattern if possible, and include all trunks you can find in there.
Cheers,
Nico