I’ve been a T-Mobile customer for a long time. In fact, my transition to T-Mobile happened when I bought my first smart phone, a Blackberry 8100 (aka Pearl).
More recently I’ve been using an LG-made Nexus 5. No, not the newer 5X. Late last year I semi-regressed from a One+ One to a Nexus 5. One of the reasons for that step backward in time was to finally be able to enjoy mobile HDVoice calls to my wife, who also carries a Nexus 5.
T-Mobile, who lead the US in the rollout of mobile HDVoice, supports it’s use for in-network calls between a list of supported handsets, including the Nexus 5. That initial rollout of HDVoice came before the big build of their LTE network. They enabled the AMR-WB codec (aka G.722.2) over their existing 3G HSPA+ network.
Most other US carriers waited until their LTE rollout to launch HDVoice. An LTE network is natively an IP network, readily supporting advanced voice codecs and video. When the voice calls are handled over the LTE network it’s called Voice-Over-LTE or VoLTE, which is very different from how voice was handled on 3G networks.