My main question is this :

Will Voice over IP work across a BM3.7/3.8 site-to-site VPN? If so, I

ideally want to link two voIP PABXs together and route multiple calls.

Next question :

How well does it work and is it a real viable solution, or should I
in some other hardware / software solution?

Lastly, if any one has any general tips regarding this type of set-up,

this would be very useful..