Most fax machines (fax software, online fax services, or fax servers) will automatically answer a fax when the call comes in.
Some fax machines and fax software have an option to manually answer the fax. In the case of fax machines, there is typically a button that says “receive” or “start” that can be pressed when the fax line is ringing. In the case of fax software, there is usually a button or menu that pops up on the computer screen when the fax line is ringing. Pressing “ok” or “start” will start receiving the fax.