The GSMK CryptoPhone PSTN/1i features a standard analog PSTN/POTS interface with an RJ11/RJ13 plug. It can also be connected to ISDN or IP networks with an optional adaptor. The CryptoPhone PSTN/1i uses the same encryption engine as GSMK’s secure mobile and satellite phones, featuring 256-bit AES and Twofish session encryption and a 4096-bit Diffie-Hellman key exchange, readout-hash based key authentication, and session keys that are newly generated for each call and destroyed after the call ends.

The CryptoPhone PSTN/1i can be switched from clear mode to secure mode and back, so that users can place clear and encrypted calls using the same telephone line.