Well I did what you suggested half-life. Hours of learning and implementing asterisk with freepbx/elastix. It all works fine on a server outside the zentyal one. I managed to get Zentyal L2TP working too - why on earth does it not just create its own network like PPTP and Openvpn?
Though everything is working, the only thing I like that zentyal voip did not have is the simple elastix call rates and billing report. A2billing is overkill, in fact, the whole thing is massive overkill, counter intuitive and lacking in sensible defaults. It is designed for big call centres with full time staff not small business needs.
I miss Zentyal voip and since zentyal asterisk was relatively lightweight on resources, prefered the inbuilt security of everything in one server - even though I use VPN now. I am probably going to try and get freepbx on the zentyal server next, but I have used it through elastix and it is even worse.The only thing I would miss is that quick elastix per user call cost report but could probably get that using excel.
Zentyal-voip was basically the only option available to anyone who did not have the time, inclination or need to learn asterisk in depth and just wanted something clean and simple that works out of the box... ie. most small businesses.