Fax and Phone on same line

Hello, I know fax is out of date, but it seems like the most viable way to communicate with my elderly grandmother. I would have no problem setting up fax in my home, I use a VOIP line converted to a standard landline and I positive I will never receive any calls on it that aren't faxes. On the other hand my grandmother uses a standard landline from her phone company all the time to make outgoing calls and to take incoming calls. If I have fax on the same line, can I set it so that if it is called by a specific number the fax answers the phone and for anyone else the fax just doesn't answer and lets ring out on her other phones. Sorry if this is weirdly worded, I'm not sure how else to explain it. Thank your for your time.