Sorry for chiming in so late. It has been a crazy few weeks. I thought I'd add a few more cents in the already constructive thread:
I believe that 600C cable is referred to as 'six conductor' because it is comprised of two twisted pairs, each with an overall shield including an integral drain wire, hence six total conductors. It is low-capacitance individually shielded pair cable.
Hint: If you can get some old IBM type 1 cable, that stuff is perfect for long T1 extensions. It has amazing capacitance and shielding qualities for such applications. It's huge (size-wise) with regard to terminations, but you can practically go forever with it.
If the service provider for the T1 has the LBO (Line Build Out) settings correct on their network interface card, then you can extend a T1 circuit up to 655 feet with minimal effort. Separately-shielded pairs for transmit and receive will certainly make the extension more stable. Keeping the transmit and receive pairs in separate cables will work, but the shielding and increased gauge to 22 AWG will be your best bet for extended distances.
Below is a link to the only modular jack designed specifically for the termination of a T1 circuit that I ever encounter. It has built-in shorting bars that will keep the circuit alive by shunting the transmit and receive pairs together when there is not a plug inserted. It effectively creates a loopback condition. Screw terminals will accept any type of wiring. Verizon has been using them around here for years and they are readily available from any supplier that carries Suttle products.
Suttle 625A28NK-2-XX
