I actually have been able to test the “Block International Roaming” switch today when in the Bahamas. This did activate IMS service and it’s simulated WiFi on my voice line for free US calling. Texting was reliable on the voice line also. This eliminates the non roaming carrier hunt to trigger “Using Cellular Data”.

I plan on posting a summary of my experiments in a single post for those interested in free US number voice service with native Dual Sim, Dual Standby phones. A US Mobile line will give free voice calling to international numbers while roaming on dual sim phones. Also, those on T-Mobile should be able to easily set up their dual sim phones for free calling to & from US numbers while roaming.