I'm having an issue with a particular site that is baffling me and I'm wondering if anybody has seen this before or has an idea about what could be causing it.

It is a Polycom VS4000 unit, using a v.35 brick, connecting to an Adtran T100 CSU/DSU, which then connects to BellSouth's video bridge via a T1 line.

As long as everything is powered up and we are not scheduled in a converence with the video bridge, we should be in loopback with the bridge. That loopback is automatically established as long as everything is powered on. That's the basic configuration for all video conferencing units in higher education in Louisiana.

But this particular unit is giving me fits. It does NOT automatically go into loopback with the bridge. But I have verified there are no problems with the network, the bridge, or my local equipment. I CAN establish a loopback connection, but I have to trick it into doing so.

I can do this by going to the Diagnostics menu on the VS4000 and selecting "Near End Loop." Once that loop is established, I exit, and immediately it establishes loopback with the bridge. Once I do that, everything works fine. But if I don't do that, it does not connect to the bridge which means it doesn't connect to any scheduled conferences.

Has anyone seen this issue before? I've swapped the CSU/DSU with a known good unit and it does the same thing. I've reseated all the cables. Verified all the settings. Everything looks good. But why does it do this?