I've been having some random gaps of dead silence during a call.
It may be related to Wi-Fi calling as I had an issue this morning trying to place a call to a local number, the call looked like it connected but there was no audio at all, so I hung up, turned off Wi-Fi calling, called back, and it worked right away.
Are you using Wi-Fi calling?
I have had issues in the past even before being on Graphene where it would work great for the longest time then act up and I would have to turn off Wi-Fi to get a good call signal. I have had the same thing happen on other Wi-Fi networks and do not see anything indicating something specific to my network.