I think you'd have to bus and train to Florence and then you can train on to Verona. It isn't especially easy to get to Florence. This article tells you how to get from Florence to Volterra and has links to timetables, so presumably you will be able to use it to work out how to do it in reverse:

…webs.com/howtogettovolterra.htm

The bus links within the link supplied by peggypatch are slightly outdated. For current bus times, scroll down on http://www.cpt.pisa.it/orari/extra.htm and click relevant lines, e.g. 500 or 790 to connect with trains at Pontedera or Cecina.

Noted - thanks.

Thanks heaps guys, we are taking the train from rome to stay in Siena and then catching a bus from Siena to Volterra for a couple days. I'd herd that getting there maybe a bit harder than other places but i will check out the links you have sent thanks

I'm pretty sure that you have to make a pretty tight change at Colle di Val d'Elsa to get from Siena to Volterra. Check out the busfox website for timetables.