Network Monitoring tool for all vendors?

We are a network management company mostly in data. We are getting clients on IP telephony (corporate voip) networks as well now. What are the best softwares/tools to monitor IPT/voip networks for all the vendors i.e. cisco, nortel, avaya etc. We want to put a software/server centrally and monitor all our different clients voip networks from that. How can we do that?