Would i require some public server side component to handle the call routing etc? (or could you just use something like Google Voice?)

I went the DIY route (you can find the details as a parent comment). But, I had good luck with voip.ms as a SIP provider. It is inexpensive at $1.10/month for the phone number and $0.008/min for calling. And it has a pretty good history of user forums, wiki, etc for debugging hints with various hardware.