SlingSecure anti spy technology is effective because it is the complete package: it is simple yet very powerful.
After an automatic initial protocol phase which performs key agreement and synchronization, the encrypted phone turns voice into data, encrypts it and splits it into small packets.
Such data packets get transmitted in real-time through the GSM data channel reaching the other device.
Upon receiving the secure packets, the other device performs data decryption, assembles the original audio and plays it on the usual phone speaker of the device.
In turn, each crypto phone device performs audio encryption and decryption in real time, so as to let the user experience a full-duplex encrypted voice communication, comparable to a normal phone call conversation.