I don't think this is possible. A leased line is a dedicated line between two points. Having ISDN on one end and frame relay on the other end is like having one party speaking Japanese and the other speaking English. They won't understand each oth...
You cannot add an Etheternet port via a WIC card on the 2621. The only way of getting additional Ethernet ports is to install an NM-1E or an NM-4E in the Network Module slot. This will give you 1 or 4 ports of 10BaseT respectively. ThanksMark