I don't know exactly when they started it, but this past week, the U.S. MVNO Straight Talk has turned on (or at least started to?) HD Audio for phone calls.
For those who are not hip to all of this...
Several years ago, cell carriers came up with a standard for sending near-CD-quality audio for phone calls, called VoLTE (voice over LTE), and carriers have slowly started implementing this. This is also known as "HD Audio" (or some variant thereof). On phones which support this standard, you can definitely hear the difference.
A Mobile Virtual Network Operator, or MVNO, is a kind of cellular service provider which does not have towers of their own. Instead, they lease time and bandwidth from physical carriers. In the United States, this includes Boost Mobile, Cricket Wireless, Virgin Mobile, Google Fi, Straight Talk, and many others. Most MVNOs pretty much stick with one, or possibly two, physical cellular carriers. Straight Talk, on the other hand, leases time from AT&T, Verizon, T-Mobile, and Sprint.
Anyhow, on Friday, one of my co-workers, who is a Verizon customer and has an iPhone, called me after work and I noticed immediately something was different about the audio of the phone call. However, it was a ~1 minute call and she isn't really very tech savvy and I had other things going, so I couldn't really check it out. However, earlier today, a friend of mine who lives out in Washington State and who is directly on T-Mobile, called me. Not only is the audio quality better, but the Google Phone app shows an "HD" badge next to the talk time.
For reference, I have a Motorola G7 running Android 9, and I use Straight Talk on the AT&T network.