How does faxing work is two questions: what happens on the phone call, and what happens when there is no machine at your end. On the call, a page is scanned into a black-and-white image, compressed, turned into audio tones and sent to the receiving device, which decodes it and confirms it page by page. Without a machine, an online service does exactly that from its own servers on your behalf, dialling the number you give it and returning the same confirmation. This page walks through both, and says what faxing a document does that an email does not.
What happens on the call
The sending device dials the number. When the receiving device answers, the two exchange a short handshake defined in the ITU's T.30 recommendation: what speeds each supports, what page size and resolution, whether the receiver can accept compressed images. Then the sender transmits the first page as tones, the receiver decodes it, and at the end of the page the receiver sends a confirmation. If the confirmation fails the page is resent or the call ends with an error. At the end of the last page the sender records a transmission report: number dialled, pages sent, time, result. That report is what makes a fax different from a letter in the post.
How does fax work with no machine at your end
An online fax service owns fax lines and numbers in a data center. To send, you upload a PDF, photograph a page in the app, or email the document to an address made from the destination number; the service converts it to a fax image, dials the number, does the handshake and the transmission, and sends you the report. To receive, the service answers calls to the number on your account, decodes the pages into a PDF and puts it in your inbox or your account. The receiving hospital's machine cannot tell the difference: it had a normal fax call from a normal number. That is why what it means to fax something has not changed even though almost nobody sending one owns hardware.
What does faxing do that email does not
Three things. The receiver's device confirms each page during the call, so the sender holds evidence of delivery rather than evidence of sending. The transmission goes point to point on the phone network with no copies held by a mail server in between, which is why medicine and law kept it. And the receiving side needs nothing but a number: no account, no attachment size limit, no spam filter. Those are also its limits: a fax is an image, not a document, and it arrives at whatever machine or account owns the number rather than at a named person. An online service keeps the confirmation and the point-to-point call and adds the inbox; the plans that include a number start at Fax.Plus's printed $6.99 a month.
Other questions people ask
How long does a fax take to go through?
A page a minute is typical at the speeds most receivers negotiate, plus the handshake. A short document arrives within minutes; the timing page on this site covers the cases.
Can a fax fail silently?
No. Either every page is confirmed and the report says so, or the report shows an error. Silence means the job never dialled; check the service's sent log.
Do I need a phone line to fax online?
No. The service's lines are used. You need a browser, an app or an email account, and a plan with a number if you want to receive.
What does it mean to fax something?
To send an exact image of a page over a telephone call to a number a fax machine or service answers, with the receiver confirming each page. Sending from an app or a web page changes only who places the call.
What is faxing a document?
Sending an exact image of its pages over a phone call to a number that a fax machine or service answers. The receiving device confirms each page, and the sender keeps a transmission report as evidence of delivery.