Fix is for broken voice and data on both the proper and Harman/Kardon versions
If you've got an HTC One M8 on Sprint, heads up — there's an "urgent" software update waiting for you. This one addresses "network connectivity maintenance and improvements" for both voice and data. And, according to the fine folks in our HTC One M8 forums, it was one hell of a bug, basically making it so voice and data wouldn't work.
The update brings your software versions up to 1.54.651.10. Best get on Wifi and get it now. It should be available for both the proper version of the M8, as well as the exclusive Harman/Kardon model.