Mark, I have many Partner systems with T-1. Most use a service from a Clec called 2 way DID. One even uses a t-1 with some channels set-up as 2 way DID and some as combo lines. I haven't had any problems with any of my installations as relates to T-1.
You will have a issue with line assignments to buttons on sets smaller than 34 buttons, if you go with combo lines. By using DID and pools you won't have that problem, or rollover issues either. Good luck