how to connect US robotics 56K fax modem on updated fiber optic line as no response from modem when call comes in

You need a physical phone line (old-fashion pair of copper wires) where to connect your modem. It won't work (or will work unreliably as fax) over VoiP adapters you might have (eg Ooma, Vonage, UVerse etc)

If you need it to receive faxes, there are plenty of online services where you can reroute your fax number and you will get your faxes over email.
 
It tends to be very tricky to get a modem to work on the newer technology. Even the adapter like magic jack or vontage that provide a analog jack and then run VoIP tend to not work. The codecs they compress the data with cause too much damage to signal for it to work. It can be done but normally it takes very specially settings on VoIP clients to support fax or modem.

There are other devices that do a form of circuit emulation that can carry uncompressed data but you need a special box on each end to make it work.