Jeff Mcadams writes:
its been our experience that once all of the channels on all of the available PRI to the ISP are in use, the telco will generate an all circuits busy type of response as the PRI are trunk-side, so the switches typically treat them as inter-switch trunks being filled, not end lines to the ISP being full.
As always YMMV. The result is affected to a great extent by provisioning. Which is obvious and yet hasn't been mentioned. One way causes a call setup request to be sent even though the switch knows that there are no available B channels, which permits the terminal to make it's own decision as to the disposition of the call, e.g., refuse it with cause code 17 which _should_ cause some switch in the call path (the originating one usually) to generate a normal busy tone.