Fax sending is not supported natively within 3CX. Its built-in fax server is receive only.
I believe that there was at one point some functionality to support fax sending, but this was a number of versions ago. I also recall that there was some support of 3rd party applications, but not having used any of them, I do not know to what degree of integration between 3CX and said application there may have been.
The only way if which I am aware for FOIP is using a gateway. You can try and set-up a normal extension and then use a gateway running g711 pass-thru or run a gateway independently of 3CX and using t38. t38 is not supported at the extension level unless it is at the system fax extension in which case it becomes receive only as 3CX itself is handling.
There are other solutions, such as analog lines, fax servers and the like, but I assumed some aspects of your issue, so may not have understood your prior set-up to provide the desired guidance.
