Hello and thank you for posting to our forum.
Any distance limitation is dictated by the type of T1 receiver interface on a unit or smart jack. Typically smart jacks are DSX interfaces and T1 network interfaces on routers/CSUs are DS1 interfaces.
T1 DSX interface - Tx level - 0dB / 6Vpp Rx level - 0dBm Length - up to 655ft
T1 DS1 interface - Tx level - 0dB / 6Vpp Rx level - down to -36dBm Length - 3000 ft w/loopback or 6000ft w/out loopback
The DS1 receiver is more sensitive and can frame up on a T1 signal over a longer distance than the DSX interface. DSX to DS1 connections require a straight through T1 cable where DSX to DSX or DS1 to DS1 connections require a T1 crossover cable (pins 1-4, 2-5).
Note that on a TA 900, there is a NET (DS1) 0/1 and DSX interface 0/2. On a TA 900e, all 4 T1s are labeled NET although T1s 0/3 and 0/4 can be configured for PRI or E&M like T1 0/2 on a TA 900. A TA 900 T1 0/2 to a PBX would use a straight through T1 cable while a TA 900e T1 0/3 or 0/4 to a PBX would require a T1 crossover cable.
Let me know if you have any questions.
Thanks for the information. This clears it up for me.