There is excellent support here on basic fax setups. You have what is known as "Distinctive Ringing Service". There are pros and cons to the various setups. I assume you recognize the rings and answer only when the ring for the landline talk comes on. The fax will pick up the faxes, and the answering machine will pick up the voice.
Need model number of your fax - google that!
Service manual for your phone.
This link shows various configurations of setting up your phone/fax/answering machine or using an answering service.
Distinctive Ringing Service setup
While it is not available everywhere, a distinctive ringing service can be purchased from most telephone companies. With this service, more than one phone number is assigned to the same phone line. Each phone number will ring differently. The main number will ring with a normal single ring and the additional number will ring with a distinctive ring pattern assigned by the phone company.
When a distinctive ringing service is used, set the Rings to Answer to 2 and then follow the steps below to set the distinctive ringing pattern:
Press Setup , press 4 , and then press 1 to enter the Answer Ring Pattern setup of the Advanced Fax settings.
- Use the Right Arrow to scroll through the available settings. When the ring pattern assigned by the phone company displays (double or triple rings), press OK .