The serial port on Cadence is basixcally what you use to program it with although you can touch tone as well.The parallel port is basically used to backup the program with the CMU utility.
As for the DID digits you mention 250 as an example.Is there a valid extension number 250 in the system? I'm guessing not, try a valid extension as in the ones in the numbering plan in mmc 724.
As for the value I have no idea what the ISDN cards are worth since the US and Canada don't use them.Also the DCS switch has been discontinued but support should be available for it for a while.I like the DCS it's a pretty solid switch.Cadence has also been discontinued for a few years now as well and I personnaly hate hard drives.The new SVMi units are available with a compact flash drive which I prefer although the storage time is reduced.Time Samsung upgraded the size of the Flash modules as the prices on those has come way down.But I digress.