Typically when you see that message it means that you're trying to send out more calls on a provider than it can handle (by the way, that's also not the correct SIP message that the OBi should be returning in that situation, since FreePBX/Asterisk sees that as a "line busy" and that prevents it from falling through to another free trunk, if one is available — I'm told that will be fixed in an upcoming firmware update, and then it hopefully should send a more appropriate type of SIP message back to Asterisk in that situation).
I've seen that message when I've deliberately tried to do something like send a second call to the LINE port when it was already in use, or send a second call to Google Voice before the most recent firmware update that bumped the maximum simultaneous calls 2. I think that it could also happen if, for example, you tried to send a call to the LINE port but the phone line wasn't connected, or if you tried to send a call to Google Voice or a SIP provider but the OBi wasn't registering with GV or the provider for whatever reason.