...
Below is all the different scenarios that can occur for a single T1/E1/J1 link. Now remember there can NOT be no clock on a T1/E1/J1 link and there can not be two clocks. For a how to on configuring multiple ports go to http://wiki.sangoma.com/wanpipe-linux-asterisk-appendix#clockSync. Also note step 4 is the typical setup when connecting to the telco.
...