TechRadar says:
The call quality and signal strength were two areas I was particularly impressed by - living in a house with very low signal gives me a great chance to test battery and call quality (as well as living a frustrating existence when I miss call after call).I found that the HTC One (M8) allowed me to make calls in new places compared to phones from 2012 and 2013, and that makes a big difference to me being able call from bed rather than having to hang around a window before having a momentary panic as to whether I've remembered to get dressed.
95% success rate, if you're wondering.
The sound quality of calls has always been good from HTC, but the rounded edges of the phone were more pleasant to press into the ear than the sharp edges of last year's model. The Boomsound speakers are smaller, but the sound quality isn't diminished thankfully.
PhoneArena says:
With its signature speakers, there’s no issue trying to make out voices in even the noisiest of environments.
As the earpiece is actually one of the stereo speakers, it really lends some help in the calling quality performance of the phone. Not only are voices exquisitely loud and commanding, but they’re for the most part clear and noise-free – making it very easy to hear our callers in extremely noisy environments. Lucky for us, that’s also the case switching it to speakerphone mode, where its dual front-firing speakers deliver a potent mixture of power and clarity. Lastly, our callers also benefit by being greeted to voices on their end of the line that are robust and lively.
Engadget says:
Our European review units fared well in overall call quality, and BoomSound helped make the speakerphone easy to use for conference calls. Speaking of which, we could tell that BoomSound was louder than it was on the M7, and music sounds clearer.
Boy Genius Report says:
The result is sound that’s deeper, 25% louder, and even a bit clearer (at least to my ear) than it was on last year’s HTC One. I should also note that sound seems to be improved on voice calls, and through the audio jack as well thanks to another new amp.
PhoneScoop says:
Regular phone calls are sent to just one of the two speakers. Calls are plenty loud and audible with the volume set between 40% and 60%. If you put it up much more than that and press the One to your ear, you might be in danger of damaging your hearing. Call quality was a little bit mixed. Most of the time it was good, but every so often I'd hear a bunch of noisy interference. Calls routed to the speakerphone are insanely loud. You can use the One to fill a board room with mostly clear voices.
PocketNow says:
Just as it was on the original One, voice calling on the M8 is an excellent experience. We tested it on AT&T’s network in wind, in traffic noise, and even in the throbbing pop-music onslaught of a German bar-restaurant in New York City. Aside from a few stray artifacts, callers reported hearing no background sound at all – and their voices came through loud and clear on our end, as well. So loud, in fact, that full volume was actually slightly painful.