Mobile device security vendors roll out an array of systems for governments and companies looking to protect their voice calls. Secusmart’s flash card with hardware-based encryption for voice calls can be installed on the phone when the card is first inserted into a device. The card is tamper proof and has been approved by Germany’s Federal Office for Information Security for use at a “restricted” classification. The company also offers a desktop encryption appliance for ISDN (Integrated Services Digital Network) lines, the TopSec 703.”]
Source: https://www.csoonline.com/article/2123789/cebit–new-options-for-mobile-voice-encryption.html