So I have IP phones on different networks all running fine. Audio is fine between phone on their own network and when they call out via trunks. However, when they intercom across the network I recieve 1-way audio but wait if I then setup a conference with another IP phone (regardless of network) audio is fine for all phones. The minute I drop the conference 1-way audio is back.

The kicker is I used the exact same phones, same networks but connected them to a 100 versus a 7400 and had the same problem.

Any thoughts why a conference call would rectify the 1-way audio?