Check with your telco, make sure that they have "BCAS" testing turned off for the span. October 2005 Qwest did an "upgrade" or something like that, at midnight the C.O. tests the B channnels, causing them to go out of service for outbound calls. I had about 15 or so systems in Phoenix with these same symptoms. On here or somewhere else on the internet it was a known issue with Nortel. When I finally was able to tell Qwest repair what to check for just one site and set the option to "no", it worked fine. Then I gave the same person the circuit ID's for the rest of the sites, and the problem was solved. The systems I had were 4.0, 4.1 and 5.0's, all same symptoms.
Until I arrived at the final solution, I'd dial into the systems and reboot to clear in/out calling. Mental health clinics not too tolerant of calling problems.