This whitepaper goes into detail on design and implementation details for performing voice encryption on telephone networks. Written in Spanish.
You can download it from the following link: https://packetstormsecurity.com/files/download/128113/voice-encryption.pdf
Source: https://packetstormsecurity.com/files/128113/Design-And-Implementation-Of-A-Voice-Encryption-System-For-Telephone-Networks.html