My own history includes fax software, back in the day.

Physical fax machine or fax software using a local computer modem, the requirements of the 'phone line' are the same. The 'phone line' must not do compression to the audio signal for a 'fax call'.

Some (many?) VOIP services either automatically detect the fax calling tone (that 2.5 second periodic beeping after it dials) or you must use some special prefix or dialling mode to tell the VOIP system that this will be a fax call and it wil need more audio bandwidth than a regular voice call.

The more reliable (and probably easier) method is to utilize one of the Internet Fax services. They come in many forms, but in all cases the service connects to the actual phone network and delivers the fax using 'real' phone lines.

You simply use their software or whatever to get the document to be faxed to the service via the Internet, then out it goes. These services tend to provide VERY reliable fax delivery once the document gets to them.

Just make darn sure the phone number you tell them to send the fax to is indeed a valid fax number. Otherwise it can be rather relentless about re-dialling the same wrong number over and over trying to deliver the fax.

Most of these services also offer (for a fee) inbound fax to Internet delivery service. They provide a phone number unique to your account and any faxes sent to that number end up in your inbox, often as PDF pages.