You need something to accept the incoming phone calls, some sort of RAS server. Maybe the easiest way of doing it is for the RAS server to also be the VPN endpoint so you run VPN between the remote PC and the RAS server. Anyway you have a few choices:
1) There are various 'solution in a box' products that will do it for you
2) If you are a Windows shop you could implement a Microsoft RAS server. You could integrate that with your domain or active directory for security and user authentication
3) There is undoubtedly an open source Linux-based way of doing the same thing (and I'm sure someone will tell us)
4) You could go for a more primitive approach if the solution is not so critical, such as a modem on a PC at the central end giving you a basic PPP connection into your site then run the VPN across that using separate kit
Depends how many remote users, how secure it has to be, how important it is that it works reliably etc
