^{[1] [2]}

Developer Krishna Sagar has found references in iOS 5.1 that refer to switching to 4G when on a phone or FaceTime call.

iDB was able to confirm ^[3] the following strings appear in file var/stash/Applications/Preferences.app/English.lproj/Network.strings

"4G_ON_CALL_CANCEL" = "Cancel";
"4G_ON_CALL_OK_DISABLE" = "Disable";
"4G_ON_CALL_OK_ENABLE" = "Enable";
"4G_ON_CALL_WARNING_DISABLE" = "Disabling 4G will end your phone call. Are you sure you want to disable 4G?";
"4G_ON_CALL_WARNING_ENABLE" = "Enabling 4G will end your phone call. Are you sure you want to enable 4G?";
"4G_ON_FACETIME_4G_WARNING_DISABLE" = "Disabling 4G will end FaceTime. Are you sure you want to disable 4G?";
"4G_ON_FACETIME_WIFI_WARNING_DISABLE" = "Disabling 4G may end FaceTime. Are you sure you want to disable 4G?";
"4G_TEXT" = "Using 4G loads data faster, but may decrease battery life.";