I have been playing with 3CX for almost a year now and I still am unable to understand this issue. There are many third party providers that sell both gsm and analog gateway solutions that can work with this platform. Many times when these devices land on my desk I conclude that for a analog gateway it will work using the pstn setting. If I am lucky, it will register but 3cx will not proceed "no routes available". After a while when i do more research most of these companies will instruct that you set it up as a voip trunk,send all calls its way and do the logic in their web interface which is mostly confusing to me with their manual. My question is in situations like this when does one identify what setup to use for a third party unit. I tried a grandstream analog gateway and i was able to have to running using the pstn setup in no time.