When we installed a couple of 6624 modules we used Cat5 RJ-21 connectors and cabling. Our local wiring contractor (River City Communications) installed the cabling for us when they were installing all of our Cat5 station cabling. We had our 24 pairs punched down on a Krone Cat5 RJ-45 patch panel. This has worked very well for us. It makes it real easy to move an analog device anywhere in the building that has Cat5 RJ-45 jack. We also made up a bunch of patch cables that have RJ-45 connectors on one end and RJ-11 connectors on the other end and we give these out to the customer that needs an analog device. This helps to prevent damaging the outside pins on a RJ-45 receptacle.
Heres a reference that I found on using Cat5 connectors and cabling.
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/cfgnotes/78_10975.htm#xtocid1221116
It seems to me that I also found information on CCO that stated the need for Cat5 and not to use Cat3 connectors, but I cannot find the page. I suspect Cat3 cabling wouldnt be a problem.