Actually all you are doing is catching channels not "lines". So relabel the park keys as lines and have them depress them and say John you have a call on Park..err Line 2. When you jump to DID numbers (yes SIP lines are DID) you enter into a new level of phone system configs. SIP doesn't have trunks it has "channels" that come from an IP source of some kind. Precisely what is the problem you are having?