An ISDN-based VTC system supporting secure (classified) and non-secure (unclassified) conferences must utilize an approved pair of EIA-530 A/B switches operated in tandem or a dual A/B switch to switch the Type 1 encryptor in/out of the circuit between the CODEC and IMUX.

ISDN-based VTC systems supporting secure (classified) and non-secure (unclassified) conferences operate in an unclassified manner while connecting a call. If the call is to be classified or "secure" at any level, the Type 1 encryptor is switched into the circuit between the CODEC and IMUX, then synced with the other end before the conference discussions can "go secure". This is typically performed using approved A/B switches on both sides of the encryptor operated in tandem.
The use of the word "tandem" here does not refer to public switched telephone network (PSTN) tandem switches. This refers to a pair of A/B switches that are operated at the same time.