I have some IX-5900 and 5910 phones that I am adding to an ECS running 8.0 (though it was upgraded from 2.1 to 8.0 prior to the MBU being installed in the system) that can make calls to IP handsets without any issue, but calls to digital handsets (or through the trunk lines) have no audio.

I've ensured that the MBU is programmed correctly in the ECS for the slot and MAC (as well as swapped in a known good MBU), as well as making sure that the MBU is updated to the latest version (1.22).

If I connect the phone directly to the MBU everything works great (I can make calls to/from TDM resources on the IP phone), but if I move the phones to the network the audio stops working.

The site in question is using Cisco switches throughout. They *do* have VLANs set up, but all of the ports are configured in the switch to be tagged with the proper VLAN. There aren't any protocols or ports blocked on the network.

Is there anything in particular that I should be looking for on the network side of things that can be causing this behavior?

Your IP keysets speak to each other 'peer-to-peer' in a manner of speaking, and the MBU is used to allow IP resources to talk to TDM resources.

If IP can't talk to TDM, something about your MBU isn't configured properly.

Verify the MBU IP address (static IP, different from LAN2 address) with proper subnet and gateway.

Double check slot and MAC programming. MAC adresses are easy to fat-finger.

Disconnect the MBU from the network and ping the MBU IP address to verify you don't have an IP conflict.

You may also want to check the network connection to the MBU, silly things like a cross-over patch instead of a straight patch cable or using a different port on the network switch.

You should see a row of lit LEDs next to the jack on the MBU for the network connection indicating the network seeing the MBU. The MBU speed settings (10/100 duplex, 10 MB, Auto Negotiate, etc) should match how the network is set up. Check with your IT guy on this.