Fax rings but machine doesn't answer

First check the setup/configuration and make sure the Fax is set to answer on 1 or 5 rings. If the line is dedicated to fax then it can be set to answer after the first ring. Fax machine works off a modem that listens for the ring and handshake tones. If the machine has been hit with a lightening spike then the hearing portion may be dead.
Translation: Your fax machine is Deaf and can not hear the ring.
It will never answer and do the fax handshake required.
If it is a new machine and still under warranty. Take it back as soon as you can.

If this is a fax machine installed at work.
Be careful, some phone systems used in the corporate buildings have too high a voltage and will fry the modem/fax card.

If this is a home machine with DSL.
Make sure the DSL line filter is applied correctly.
It will be clearly marked Phone or DSL line. Do NOT connect the phone/fax line into the DSL line connection.

Hope this helps!
Good luck!