I've had several phone company guys tell me that CAT3 is fine for T1, but they recommend splitting the pairs between binder groups. I just finished one install (frac T1 for data) where telco put the smart jack off the demarc with about 10' of regular old cross-connect, then to the demarc extension (about 350') over 200-pair CAT5. We then went from the 66 block at the extension to the rack with about 75'-100' of CAT5e. It even works!