Do not know as you do not state how many calls you anticipate making at any one time. You also did not state the memory or OS that the system will have.
I will be running windows 7 ultimate with at least 3 gig memory(can put in 4 gig if needed) and have 80 gig hard as main and can install second if needed
I ran a few lines through a ThinkPad X31.. I'd expect this machine to handle it okay.
If not, this model is on eBay for under $30.. I'm betting you could scrape up something with more umph. Considering the price for 7 Ultimate which a 2006 model would not have come up with, the OS is more than the machine, even through non-profit/education channels.
That X31 was the vanilla XP variety.. handled v10 okay back when I was using it.
Not to disuade you from using 3CX or troll your thread.. but consider a hosted PBX for a single SIP trunk and a PSTN.. seems that the system may be overkill.. while the hardware is undercooked.