I have to agree with NFtoBC in that a switching box of this nature doesn't exist commercially, at least not to my knowledge. His suggestion to have the fax pick up after more rings is a good idea. I'd also suggest that you double-check your manual to be sure that there are no workarounds for this situation, as I'm surprised to hear that your fax doesn't support Smart Ring. Worst case, we can cut your fax number over to a separate line that would be dedicated to the fax. If you'd like to speak with someone about that, please send me a PM with your contact info and I will set up a call with a representative.
I think the phone line still supports smart ring when delivered via the fibre.
Also I see nobody gave the answer to the original question.
Yes someone was making boxes that direct calls based on smart ring.
A website that retails these: http://faxswitches.info/index.php/