IP-based VTC systems must not connect to ISDN lines when connected to a classified network.

Most CODECs have a built-in IMUX such that multiple ISDN lines can terminate directly on the CODEC. ISDN lines used for VTC transport are provided by a traditional telephone switch on an unclassified network. Connecting a classified IP network to an unclassified telephone network through a VTC CODEC while in a conference could lead to disclosure of classified information to the unclassified network and unclassified VTC endpoints. While this issue might be mitigated by using a Type 1 encryptor between the CODEC and an external IMUX, an SOP would need to be in place which would dictate that the ISDN connection must be established and the Type 1 encryptor synced with the other end BEFORE the CODEC was connected to the classified IP network. This type of operation is risky and prone to error and is therefore not recommended.