Your iPhone will let you upload dictations via the Wifi network that you are already connected to. In other words the Dictation+ app simply uses the Wifi connection that is active at any given time.
|As an option you can chose to also upload dictations via your mobile data connection. You will need to set the Upload via mobile data setting in the Settings menu of the Dictation + app, to On.
If the application has access to both Wifi and mobile data simultaneously, it will default to use the Wifi connection.