Asterisk runs under Linux on a standard PC. If you don't have knowledge of both, your new client would be better served by someone that does. (Man, I hate to be a bearer of bad news! frown )
That said, Linux and Asterisk are open source programs.(FREE) If you have a spare PC (Doesn't take a whole lot of memory or the fastest processor) you might want to start studying. Asterisk and Linux are not going away. Periphial cards are on e-bay and other places.

When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.