Using a VOIP 'line' for outbound fax delivery (direct from your home to the receiving company) can work, but there are technical requirements and some restrictions. The actual FAX communications protocol is ancient 1970 era tech, with a few modern things added on. Oftentimes none of the modern stuff is actually utilized on a given fax call so the 'old' protocols are 'relied upon'.

The first requirement is that the VOIP service and your actual ISP service to your location be really consistent. No packet dropping, no highly variable packet delays, just a clean IP stream upbound towards your ISP.

You really want to have a modern APA box that can understand a fax connection and provision the VOIP link appropriately for the fax call. This means that the voice compression methods will be bypassed for the duration of the fax call and that the fax link data rate to the far end machine can be faster. The faster data rate reduces the chances of a really long fax (100 pages!) suffering a disconnect part way through the call.

If you are fortunate with your selection of faxing software on the PC you can enable fax Error Correction Mode, which ensures that the fax pages are either delivered with 100% fidelity* or the fax pages are re-sent until correct or something gives up.

* Of course often this setting is buried or unavailable, so you may have no control over ECM. And may not even be able to tell whether it was used or whether errors occurred (and retried).

If using an actual physical fax machine, typically buried in a settings menu somewhere you can select ECM and also select the maximum send data rate. Reducing the maximum send rate will prevent the fax machine from attempting to use likely-to-fail speeds. Decrement the max allowed sending 'baud rate' until you can reliably send test faxes.

Actual fax data rates while using VOIP can vary widely. I have seen faxes dribbling through at a measly 2400 baud (or slower), which can take hours to complete sending of a long fax document. Some fax machines will time out at slow data rates and hang up even though the pages were slowly arriving at the far end.

ECM mode will add a little more time to each page sent but at least you will know they got there intact. ECM is worth it when faxing in difficult conditions.

The built-in Windows 'Fax' printer thing was 'workable' back in the day with directly attached PC fax modems and real phone lines. I have not looked at it with any interest in nearly 20 years. No idea what it requires technically nor how well it works, never mind whether using a 'fax modem connected via VOIP' would undermine it.

WinFax is long gone from the market. Have not paid any attention to what became of our former competitors, one of which eventually bought WinFax.