With my experience with T1's (I've done a few, just plant cross connecting) it doesn't even have to be category 3. We've ran T1's on 1800 pair pre-category 3 cable, where it was I want to say either 24 or 26 guage. We always seperated binder groups because the pair twist was less.

I usually see T1's done around here with two pair shielded category 5 cable. T1's aren't anything special, they don't need that... but I think it prevents interference in the customer premise more than anything.