I have the same experience, solved it by deleting the "CallHistory" files and restarting FaceTime. The problem is that the corrupt files get transferred to the iPhone via iCloud and cause disconnects from the VoLTE IMS server until the phone is restarted so you end up disconnecting from LTE to make voice calls.
Go to ~/Library and search for "CallHistory" and click "Library"
You will see the files...
~/Library/Application Support/CallHistoryDB/CallHistory.storedata
~/Library/Application Support/CallHistoryDB/CallHistory.storedata-shm
~/Library/Application Support/CallHistoryDB/CallHistory.storedata-wal
~Library/Application Support/CallHistoryDB
~Library/Application Support/CallHistoryTransactions
~Library/Application Support/CallHistoryDB/com.apple.callhistory.databaseInfo.plist
~Library/Containers/com.apple.accessibility.heard/Data/Library/Accessibility/TTY/TTYCallHistory.sqlite
~Library/Containers/com.apple.accessibility.heard/Data/Library/Accessibility/TTY/TTYCallHistory.sqlite-shm
~Library/Containers/com.apple.accessibility.heard/Data/Library/Accessibility/TTY/TTYCallHistory.sqlite-wal
Delete all the files and empty trash.
Restart FaceTime.
Here are some keywords for google so people can find this post:
AT&T iPhone VoLTE IMS disconnects
AT&T iPhone IMS connection is lost
AT&T iPhone connectivity problems
