IMO you should just say ‘no’ to the fax machine. 90% of what I see put into fax machines came out of the printer just minutes before, usually a form that they filled in and/or signed. Learn to do this with software, use an inexpensive virtual fax service instead, and save some trees.
If the document to be faxed is not already on the computer (it came by postal mail or was delivered personally), it’s likely something important such as a real estate contract or insurance policy, where you would want a permanent record and scan it anyhow.
If the customer is stuck in the 20th century and insists on a fax machine, just say ‘no’ to T.38 and other flaky FoIP schemes. Get an HTTPS-based adapter such as https://www.nextiva.com/support/articles/setting-up-a-fax-bridge.html . You don’t have to configure the device – it’s plug and play. You don’t have to configure the fax machine – it negotiates baud rate and ECM automatically. You don’t have to configure the PBX – faxes bypass the PBX altogether. And you don’t have to configure the firewall; if the adapter can pull an IP address, do DNS lookups and make outbound TCP connections to port 443, it just works. Similar devices are available from Faxage, Vitelity and probably several others.
Yes, these services cost a bit more, but life is too short for failed fax headaches.