I take it that you are using an 'aftermarket' encryption module, or simply one that is software driven, like many iterations of DES, or AES-256.
I deal with two way radio encryption modules that are for commercial radio systems which use a physical module installed inside the radio, then you connect a device called a 'keyloader' to program the encryption 'key', and then upload the entire sequence to the radio.
I know all user-defined encryption for phones is strictly for data transmission, and any type of voice encryption is handled by the carrier, as it would be impossible for every handset maker to design compatible encryption devices that work with every model of phone, as well as modulation type, and there are many.
As your encryption 'device/module' is active, it suggests this is an actual module, deriving power from the phone's battery, which will place a constant drain on phone resources just for thetask of providing end to end data encryption.
All active devices that are tasked to be in a constant 'on' state should draw a minimal amount of current, but not so much as to be a heavy burden on the battery life, being commanded on and off as necessary, and being placed into a standby mode when not required.
I am really interested in the type, modle and encryption level you are using, and what code string is being used to encrypt your data.
If this is an installed device, does an icon appear on the display, indicating current activity of the device?
So many questions, so little time....My life revolves around radio, and security features are a topic I have been involved with for close to a decade.