What does it mean if a fax recipient has a single fax line?

You may see an error message returned for ringing time out error or the fax will not be delivered as they have a single fax line with their telco service.
This means that “a single line fax is a POTS line which means the fax line can only accept 1 call at a time. When someone else tries to send a fax to the destination number it will receive a busy signal because the line is already occupied.
To resolve, you need to attempt your fax again at a time less busy with the recipient not receiving other faxes at same time. Or the fax recipient needs to discuss the issue with their carrier and request for their fax line not to be a “single” fax line so they can receive more than one fax at once or on a queue.