My fax goes off periodicall when a call comes in.

It means that your fax machine is "answering" the incoming call before you pick up the handset. Usually, the factory setting on the fax machine picks up after two incoming rings. There are three ways you can address this issue: 1.) You can either change the number of rings to, say, five incoming rings to give you enough time to answer the phone before the fax machine picks up, or 2,) you can find out if your fax machine has a feature that allows for the voice hook "click over" that will allow you to override the fax when you pick up your handset, or 3.) turn your fax machine off until you need to send a fax.

If you want to investigate number 2.), then you'll need your operator's manual.

Good luck.