I am right in the middle of developing a smart call routing system for our company to save calling costs with our employees travelling (abroad). The easiest way would be to do some mix-up between standard fixed-line/ mobile and VoIP since 3G internet-only SIMs are available in many countries our staff travels to. Of course smart call routing could also be used in your home network when calling destinations abroad.

Could it make sense to implement some sort of dialplan so that the phone decides by itself whether to dial via mobile or the SIP account/Skype?

With that just being said, I'd like to hear your thought s of that idea. Go ahead, rip it apart ;-)