Audio / Video Conferencing using MSN & I

We have two offices. We want to implement Audio / Video conferencing between both offices. Both office are using MS ISA. We also have VPN conectivity between the two. But its not always on connectivity.

My question is is there a way to configure Voice Conversation of MSN messenger between two offices, while maintaing the security.

What about using ISA Gatekeeper then what would be required to implement it?

MS Exchange2000 supports MSN for messaging. Since both office have their own exchange systems (We are also using the Head office exchange) so is there any scenario of using it with ISA gatekeeper?

The solution should be reliable, safe & secure.