This app was mentioned in 7 comments, with an average of 1.86 upvotes
I gave you an upvote for mentioning the app, but he's using AT&T Unified Messaging, which combines landline and wireless voicemail using the landline VM platform, which isn't supported using the app you mentioned. You'll instead need this one. Also does voice to text transcriptions.
Source: AT&T employee, who used to work on the only wireless team that handled these.
You can only link your Google Voice number to ATT, meaning that they would dial your GV number, but if you don't answer it would go to your ATT voicemail. Here's the ATT voicemail app, it will work:
https://play.google.com/store/apps/details?id=com.att.um.androidvmv
If you're using your Pixel on AT&T just download the AT&T visual voicemail app (https://play.google.com/store/apps/details?id=com.att.um.androidvmv&hl=en). It's a secondary app, but at least you can have visual voicemail.
You can't download the AT&T Visual Voicemail app? They have an app on the Play Store. If it's not working with the Nexus 6P yet, it's because AT&T is slow to update their apps. https://play.google.com/store/apps/details?id=com.att.um.androidvmv&hl=en
Is this what you were looking for? https://play.google.com/store/apps/details?id=com.att.um.androidvmv
Put it on the phone via Play Store. https://play.google.com/store/apps/details?id=com.att.um.androidvmv
You probably have to install the AT&T voicemail app
https://play.google.com/store/apps/details?id=com.att.um.androidvmv