My Nokia Lumia 520 running Windows Phone 8.1 Update produces a system alert sound whenever I complete a call (because I am on a pay as you go plan - the alert text indicates the cost of the call). The alert produces a sound that is the same as the sound named "Nokia message." Unchecking "System alerts" in Settings -> ringtones + sounds does not prevent the alert sound. Changing the sound set in Settings -> notifications + actions -> Phone has no effect on this particular message. I haven't been able to find what setting to use to change the sound played for these alerts.
How can I silence or change the alert (or message?) sound?
(The alert/message is not a text message - a record of it does not appear in the Messaging app.) (Carrier is Airvoice Wireless MVNO on AT&T network.)