First I configured the device to use inside my 3CX pbx phone system to connect analog phones to the system. This was fairly straightforward especially with the notes provided on the 3CX blog. After following these steps the device showed a status of REGISTERED in the device configuration screen as well as in the 3CX console. This enabled me to connect an analog telephone to function as an extension on my VOIP phone system - it could send and receive internal extension calls and send and receive calls with phones outside the system.
Next I wanted to see if I could use this device to connect a fax machine to send/receive faxes over a VOIP SIP service. I have accounts with a few SIP services and I tried the connection with my primary service from NexVortex and also with another service from CallCentric. These services both support T.38 which is a method for sending fax data over an internet connection. I'm not going to attempt to explain this but suffice it to say that you need at least this to have a hope of sending a fax over the internet. Even so, this is not a guarantee that it will work in every situation. There are many variables involved.
In any event, my limited test did work for sending a fax and receiving a fax. By the way, for this test, and to keep things simple, I configured the Grandstream 286 directly to the SIP provider - bypassing the 3CX server. This required some network configuration changes to route the SIP traffic from my router directly to the Grandstream. I used an available DID which routed directly to the Grandstream.