I just put in such a link, and checked before hand. In the past we have just extended the dmarc with Cat5, I've not had any problem. However on this install the customers computer equipment vendor specified actual DSX-1 cable. The cable is two pair, individually shielded.
Now..termination. On this job I had several different vendors on site as we were connecting some wireless T1 equipment to a PBX t1 card. There were many opinions on how to terminate. What the two vendors agreed on finally was to not used the ground at all. So we ended up terminating the shielded cable on modular RJ-48 jacks, and letting them connect their equipment to them.
Personally I don't believe that to be very effective, the shield should be terminated and grounded on one end at least.
You might very well get the T1 to carry just fine on Cat5 at that distance. The T1 (depending on configuration) will use pins 1,2 for one direction, and 4,5 for the other direction (or maybe 7,8). So you can see by those pin numbers that a standard Cat5 cable keeps tx on one pair, and rx on another pair. However, DSX-1 signals are very picky about balance. Newer Cat5e has different twist rates for different pairs. In short there could be a balance problem, though I have not experienced it.
Also on some installations, I find it just easier to relocate the telco's equipment on regular telephone wire and have the actual demarc where I want it.
There must be more than one opinion or method

Daron J. Wilson
Telecom Manager
LH Morris Electric, Inc.
daron.wilson@lhmorris.com