Is it turned on in the iPhone settings, phone section?
Networks can enable disable it for specific phones and even specific firmware versions. The parameters are also configurable by the network - whether to favour WiFi or conventional signal.
With iPhones, the only thing for certain is if it lets you turn it on in settings, or if you can even see the option to turn it on, the network allows it. If turned on, it will use WiFi if/when it decides to. A sure test would be to go somewhere with no standard signal but good WiFi. Maybe hide in a cupboard near your router....