Android 8.1 Oreo started rolling out to Pixel and Nexus devices earlier this week, and especially for devices that aren't the Pixel 2, there's a lot of new stuff to check out. However, if you're using a Pixel 2 or Pixel 2 XL on AT&T, there's a new feature included just for you with the latest update.
When updating your Pixel 2 to 8.1 Oreo, you'll see that it includes HD Voice Calling for AT&T users, in addition to everything else 8.1 has to offer.
Once you've downloaded and installed the new software, you can turn on HD Voice Calling by going to Settings → Network & Internet → Advanced Calling. Once Advanced Calling is enabled, HD Voice will be used for both parties of the call (assuming they're using compatible hardware) in areas where the service is supported.