CO ports are what you would plug an FXS channel into ( think of an fxs port to be a phone line you would get from the telco ) and to attach to your extension ports you need FXO ports. 2 Sipura SPA-2000’s will not achieve the application that you are trying to create here. The sipura SPA-3000 has 1 FXS port and 1 FXO port integrated into the same unit. Here is a good explanation of the hardware you need:
only extension ports are available for interfacing to the PBX. At this site you need to have an FXO device ( one or more ports ) that you can tie into the PBX as if it were another extension. You MUST be sure that the extension you are trying to interface to is not a digital system ( i.e. Nortel or panasonic or any larger systems ) unless you use an ATA to convert the digital port into an analog port.
You can choose whatever hardware you have availiable ( your SPA-2000’s are a nice choice here ) the SPA-2000’s must be tied into the PBX as if you bought more lines from the telco ( CO lines ). When your “remote users” dial these FXS lines, the associated CO trunk line will ring into the PBX.
Extra stuff needed?? yes.
you need a SIP proxy to manage the registration of the sip devices. I personally have had good experience with interactive Intelligence’s interaction sip proxy ( it’s 200 bucks ). There are other products out there that I know are free but a quick solution with a 5 minute install is worth it for me.