Even when my ringer is disabled, my phone beeps when I miss a call. This is disruptive, particularly when I'm in class. How can I prevent this beep from occurring?
I am going to assume that you have a Verizon HTC 8X - this is a known issue and you need to wait for Verizon's update:
"After further review this is a known issue with the HTC 8X. It is under review by HTC and the issue will be addressed with the next software update. Visit http://vz.to/n5snpq for the latest news on software updates and other device offerings"
Also note that there is a settings on HTC 8X:
At the bottom of the ringtones + sounds setting is a an option to disable All other notifications, this should disable the missed calls alert. However for Verizon the settings will have no effect.
Also read: Verizon 8X missed call beep
I found the solution in Settings -> Accessibility -> Reminder of Messages. When this is on, it will remind every 15 seconds (or max 5 min).