*A VOIP line can be used for analog devices if a Cisco SPA112 is installed.* Cost is $160.00 and is requested as you would a standard phone.
First find out if the line ever worked. Then confirm what equipment is using the SPA112. It may not be compatible. SPA112's are being used for many analog applications, not just FAX machines and the same reset works for all. i.e. Cordless phones, Bi Amp Tesira Audio Conference set ups to name some. It would be ideal if the client had an old straight up analog phone and could test the function before and after reset because if they can make an outgoing and an incoming call with just the analog phone plugged in, then the SPA112 is fine and the problem is either their cord or equipment.
Fax machines need these settings: http://support.t38fax.com/support/solutions/articles/19000026933-recommended-fax-machine-settings
If having issues, the adapter can be reset by doing the following:
Press the RED reset button on the Cisco adapter with a pin and hold it for about 20 seconds. If you don't hold it in that long, it will not reset.
The LED lights will start blinking for a good 10-15 minutes.
Once the blinking stops and all 3 LEDs are lit, For a fax machine, call the number with a phone to verify you receive a FAX tone.