Ha! A little trip down memory lane with the HowardForums link. I am still using the IMS function and it is a solid system for travelers who have dual sim devices. I was recently in The Bahamas and was able to make free calls with IMS between two iPhones while both were roaming there. I wanted to check this function by experimenting, and it works well. It seems that if both US phones are using the IMS configuration, then they can call each other anywhere in the world as if both are on WiFi without charge. This means you can leave your phone on as you roam for free cellular calls to and from US numbers.

I did also discover that picking the US First Responder Network (313 100) as the voice line non roaming carrier, does continue to allow the default to your data line from anywhere. This could be important because you could visit a location that only has one carrier, and if it is a roaming partner, IMS will not function toll free. As long as 313 100 is set prior to leaving it's available selection area, it will continue to "Give" no service for using data as a pseudo WiFi carrier. One small caveat, if you reset your Network Settings, you will lose your network selection and default to Automatic Network Selection, eliminating the free call configuration.