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?

Sounds like the conference call is using the MGI card. If a call is made from IP phone to IP phone, the system will do the call setup but then the voice will be routed directly, bypassing the system. This works fine when all IP handsets are in the same network as the phone system, but will often cause one way speech issues when handsets are at different remote locations due to data being blocked by what seems like a correctly programmed router.

Easiest solution is to set the IP extensions to "Use remote MGI". This way the voice traffic is being routed via the MGI. It will take up an MGI port for the duration of the conversation, but usually fixes the problem.