A secure phone line would indicate that there is encryption in place. This would be similar to how we do encryption in networks using TLS, etc. The line on it's own would not typically be encrypted, but the data flowing over the line would be encrypted. Typically, there is encoding software/hardware built into the phone units or you could encrypt the data using an external device that then goes over the phone line. If this is a digital phone system, the encryption can be just like TLS, SSH, etc. You can have mutual authentication, a lookup table for the shared key, etc. It is also possible to do "analog encryption" by doing modifications to the carrier signal and demodulating them on the receiver's end, though analog encryption is typically more complex to use and frequently requires synchronisation since you are not dealing with discrete chunks of data (see scrambling).
You should know ahead of time if you are using an encrypted line or not. There may also be other considerations such as securing the physical medium to limit the ability for someone to tap the line and know that any repeaters, switches, etc are secure. If the line is secure on one side, the other side would hear gibberish if they were not in secure mode. Assuming you had a phone system where you could switch from secure to not secure, if both sides forgot to turn on the encryption it might be a valid question, but if at least one side was encrypted, it would not be able to send or receive intelligibly (if you try to decode unencoded information, it will be gibberish). The last consideration would be if the message was encrypted end-to-end. It is plausible that within "a trusted network" the message is passed decrypted, especially in an analog system, to reduce the possibility of timing delays, etc. Based on the classification of the communication this may or may not be acceptable (require greater assurance if you are talking about a mole).
It is possible to say that a cellular connection or phone system is encrypted. GSM and CDMA have encryption capabilities; however, if you do not encrypt at the two endpoints, then you leave opportunities for interception, and the encryption of the network may not be sufficient to meet your needs.
- Further reading:
- Secure Voice at Wikipedia
- Automatic Secure Voice Communications Network <-US Government's Secure Voice Network until about the 80s
- Defence Red Switch Network / Multilevel Secure Voice <- US Secure Voice Network
- DISA Info about voice services
