Why does my fax machine ring and ring but does not pick up when someone is faxing me

Again..no make or model...however ; if it is in the FAX mode then the next thing to check would be to see how it is set to answer( they can be set to answer on the 1st, 2nd, 3rd, 4th ring etc , etc.) again ..not knowing make or model cant help any further. Again..no make or model...however ; if it is in the FAX mode then the next thing to check would be to see how it is set to answer( they can be set to answer on the 1st, 2nd, 3rd, 4th ring etc , etc.) again ..not knowing make or model cant help any further.