EXTREMELY IMPORTANT AFTER-THOUGHT!!!!!!!!!!!!!!!!!!!
I want to ensure that we are on the same page here: in your V3 system, is your STMI board actually a 60 or 120 B-channel STMI2? Or is it the original 30 B-channel STMI?
If your HiPath 4000 V3 is a STMI2, and your V5 & V6 STMI boards are STMI2 or STMI4 boards, then my previous answer stands.
However, if your HiPath 4000 V3 board is the original 30 B-channel STMI, then you are using IP Trunking V1, which uses a different "stack" compared to the IP Trunking V2 which uses the STMI2 and/or STMI4. Both versions of IP Trunking (V1 & V2) are supported thru V5 - not sure about V6 yet - will check on that today. But if you currently use the 30 B-channel STMI today in the V3 system, this changes my answer. An IP Trunking V1 board cannot communicate directly with an IP Trunking V2 board because of the stack differences. There are two solutions for this scenario. The BEST solution will be for you to abandon the original 30 B-channel STMI and purchase STMI2 for the V3 system. You will then be running IP Trunking V2 between all systems, including V6 - works beautifully. Second solution: if you retain the original 30 B-channel STMI, then you are technically running in "Mixed Environment" mode. If you have three systems, then you will have your original STMI in the V3 system, and you must install a second 30 channel STMI in the V5 system (yes, this is supported in V5). If you are attempting to call from the V3 system to the V6 system, then your call must travel from the V3 STMI to the V5 STMI using IP Trunking V1. Then the V5 system will convert that to TDM, then send the call to a STMI2 or STMI4 in the V5 system and then onwards to the V6 system. Why? There must be a point of conversion between IP Trunking V1 and IP Trunking V2 - in this example this conversion happens in the V5 system via TDM. Difficult to explain, but it does work this way.
I HOPE you have STMI2 in your V3 system so that you can avoid this scenario. Please re-post so I don't have to worry about my previous response!!!!!!!! I can send documentation to further explain "mixed environment" if necessary. Thanks