[Serusers] Newbie question: asterisk and ser

Hi All, Is my setup possible? Or maybe the right question is "Is this correct?" 1 test server: installed ser and asterisk(didn't really understand much of it yet). 1 CISCO 1750: with 2 FXO. 2 UA's: X-Lite Using SER only, I can make calls between extensions. Using Asterisk alone, can I also call between extensions? And is an extension on asterisk different on the exetensions at ser? How can I make an extension in SER also an extension in asterisk? Is it possible to route PSTN calls to the CISCO 1750 via asterisk? How about via ser? If both are possible, which should I use then? I've read somewhere that combining SER with Asterisk, or vice versa, will be very good, how come? Regards, Ron