3CX only supports receiving when used in conjunction with a provider and equipment that supports the t38 protocol. A small number of users have reported some success in getting outbound faxes to work, but these are seemingly the luckier ones. Do a forum & blog search and you can see the various issues and how some have worked through them. It is not a 3CX issue per se, as much as it is simply a technology issue that has to overcome a number of variables and obstacles. If you elect to try, do yourself a huge favor and stay with supported (recommended) devices and vendors (that support t38 and can fall back to g711).