The end of ISDN has been predicted for years - especially with the rise and rise of ipDTL, Source Connect, Session Link Pro etc.
However ISDN is still very much alive.
If you have a feedback loop it is almost impossible for the voiceover talent to talk while listening to it. This is because they will hear themselves repeatedly echo-ing.
This problem is caused because both ISDN studios need to monitor and send audio using the same line. So how do we stop this problem?
If you use an audio mixer, you need to create two separate and independent "mixes": one for yourself and one for the other ISDN studio.
Set up your audio mixer to send a "Clean Feed" (or "Mix Minus".) In other words you need to send all your audio to the other ISDN codec but don't send the audio of the other ISDN studio back to themselves. This will eliminate the feedback loop.
The only time that ISDN delay would be an issue is if you have an audio or video file being played from the remote ISDN studio.
The delay is normally in milliseconds and so would not be an issue for most voiceover sessions.
If it does become an issue, then ask the producer at the other studio to send the audio or video file to the studio where the voiceovers are located.