Add Option to Override ENUM Toll-Free Provider

[ctylor]

I just noticed that even if I specify it in the dial plan, it is not possible to use another provider than tf.voipmich.com to do North American 1-800 toll-free numbers. It would be really nice if there was some way to override voipmich.com ENUM monopoly on 800 numbers for phoning either specific toll-free numbers or to in general to substitute a completely different provider in place of voipmich.com. I know Gizmo Project (proxy01.sipphone.com) and FWD (fwd.pulver.com) allow toll-free calls to N. America through them, and it would be nice to be allowed to configure them to be the provider if we want.