Which carrier has the clearest (HD) voice when calling someone?
Are you in the US? All four major US carriers have some sort of HD voice going on. AT&T, Verizon, and T-Mobile all offer HD Voice using VoLTE while Sprint offers their version of "HD" voice over their 3G network as they do not have plans to offer VoLTE. Because of that Sprint iPhones do not have access to HD voice unless Sprint changes their course and joins the VoLTE bandwagon.
Technically speaking, the "quality" of the HD voice is dependent on the bitrate and codec used to encode the voice stream. T, VZ, TMUS all use the same codec, but a different bitrate. T-Mobile has the highest bitrate at 24.4kbps while T/VZ are at 12.2kbps. There has been some discussion about this topic and the consensus is that the difference between the two bitrates isn't all that noticeable and both sound quite good. Both T/TMUS have VoLTE turned on by default on the iPhones while VZ you will have to go into your online account manager and enable the feature as well as turning on VoLTE in your iPhone's settings. Because not a lot of people know about HD voice on verizon, you will have a hard time making HD voice calls because both parties need to have HD voice enabled. So in order to have the best experience with HD voice, I would suggest either T/TMUS as your iPhone will be defaulting to use VoLTE and most other users with VoLTE phones on the network have it enabled. TMUS has a slight edge over T as they have many more phones with VoLTE and have been seeding their customer base longer. On a side note, you can only complete HD voice calls within your own carrier's network so the call with revert back to standard voice codec when calling someone on another carrier. The carriers are working on interoperability, but who knows how long that will take. VZ announced in the beginning of 2015 that they would have interoperability with T, but it is now 2016 and is yet to be seen.