I am contemplating getting an E1 line for TDA 600 to give DID feature to all extensions, and while searching for the right card for this I saw DID card that I think works with normal trunk lines? any one can shed light to this, what this card does and what it needs to work? and whether I need other than the E1 card itself to make DID work i.e should I persue the E1 line option? thanks!