The Mapillary app (for iOS or Android) is 100% free and does not require any data plan or cell connectivity when taking images as it uses your phone’s GPS to determine the location of the images. The app defaults to uploading your images when you are connected to WiFi, not using your mobile data. Once you go on WiFi your images can be uploaded and combined with everything else on Mapillary.
The only thing that uses data while capturing is the map (if you turn it on). If traveling, you should turn off data roaming to make sure zero data will be used.