HTC One M8 User Manual page 19

You need a 4G/LTE plan to use your operator's 4G/LTE network
for voice calls and data services. Check with your operator for
details.
§ If you want to use a case or screen protector, don't cover and
block the proximity sensor. Purchase a case or screen protector
designed for HTC One.
§ Avoid connecting third-party headsets or accessories with
metallic charms that dangle near the headset jack. Using these
may affect signal reception.
Unboxing 19