Just to clarify things, I was only concerned with sending faxes out via my VOIP provider, not receiving.
My final solution to the problem was to create a second account with Callcentric and configure my SPA-2102 to use that account directly, there by taking 3CX out of the mix entirely.
I was able to make this method work with an old Panasonic fax machine and a newer HP MFC.
It also worked with a Linksys SPA-8000 and a Grandstream HT286.
All of my inbound faxes are handled by Callcentric and are sent to my email.