Originally Posted by draver

The basic configuration is, on a dual sim, dual service device your primary voice line needs to be set to a non roaming carrier so that it receives no service. On T-Mobile in the US under Cellular-Network Selection, I choose 313-100 which is the First Responders Network on AT&T. Your second line needs to be set to a carrier with data service. The concept provides that the voice line with no service defaults to the data line as a pseudo WiFi service and connects as if on a WiFi network. On dual sim iPhones the Status Bar displays "T-Mobile Using Cellular Data" instead of a carrier name. WiFi should be turned off as a precaution in case you might get a temp WiFi connection, which could be a chargeable call.