Connecting different types of PBXs together

Let's say a prospect has branch offices around the world and wants to implement VoIP to cut toll charges for international...

calls and conferences; But, the existing PBXs in different countries have different Vendor's models and at different release levels. (i.e. Nortel Meridian, AVAYA, Siemens, etc.)

The solution constraints: using minimum add-on and upgrade, over the public internet, with a VPN box tied back via ITG to each of the PBXs. Plus one call manager at HQ.

Questions:

1. Would you recommend this approach, considering the unknown problems of tying together PBX equipment and protocol implementations from several vendors?

2. Could you suggest an alternative minimum cost solution approach?
Thank you.
As long as you are using a PBX from a major vendor (as you indicate in your list) you will probably have fewer problems that you expect. Many people are connecting different types of PBXs together using VOIP to save money. What is becoming more common is that companies are finding out that they can save a lot of money by consolidating PBXs to reduce the vendors used. The primary savings is reduced consulting time needed from PBX vendors and reduced PBX maintenance. It's expensive to keep trained staff on lots of different PBXs or to pay the PBXs vendors to work on the PBXs.

I'd suggest you look at 1) how to consolidate PBXs (or reduce the number of vendors) and 2) start a small VoIP migration by interconnecting several of your PBXs with an IP VPN - the several sites should be picked based on who will have the greatest savings in switched voice costs.