My best guess is it might be a glitch with the Bluetooth connection. Tech gremlins can be real stinkers sometimes.

Here's what I'd try: toggling Bluetooth off and on again, restarting your phone, or forgetting the Anzu device and re-pairing it. If those don't work, maybe a visit to the Razer support site or app (they have a great "Who Called Me" feature, by the way)  could shed some light.
In the meantime, channel your inner zen.  Anzu calls are supposed to be a smooth experience, so hopefully it's a one-time thing.