This is interesting. We have just had something very similar to this start happening to our system. We have 4 lines and yesterday one of the ladies told me that no one could dial out. I check and sure enough we were getting the "No Lines Available" message. I checked the gateway and found that all 4 lines were listed as TALKING, but only 1 line was actually in use.
I ended up rebooting the gateway and it seemed fine for a while, then it happened again today. We have been using this system for months with no issues like this, so this is strange.
We have all of the latest SP's and firmware installed.
Any ideas???