Session Terminated - what session?

Since July 2018 my phone has shown the below image 5 or 6 times. It happened again today when I was on the phone to the Commonwealth Bank. I was at the beginning of the conversation when the automated service was asking me to select a number to signal which department I wished to talk to. When I selected a number the screen above appeared. The number would not select and I had to hang up and switch my phone off. I turned the phone back on and then I was able to select a number.

Any ideas what this "Session Terminated' relates to?