Fax, unfortunately, is one of those things that will keep a lot of companies from going completely cloud. And, as Bert said, fax just isn't going anywhere. Even to this day, GFI's (
www.gfi.com) sales of their Faxmaker software is solid. As is their sales of their MailEssentials antispam for Exchange as there are still plenty of on-prem Exchange servers that just aren't going anywhere.
That being said, for rock solid performance, Bert and Sandeep are correct about Mainpine. I would also throw in GFI Faxmaker with a BrookTrout board. I've have clients that have Brooktrout cards that are now 10 years old and they still chug along. You can dump faxes into either a folder, an email box on your Exchange Server, or a public folder on your Exchange server.
Sending faxes is as simple as installing the client and printing to it via Word or whatever (the client sets itself up as a Windows printer). The client will prompt you for a phone number. Or, if you have an Exchange server, you can send an email to xxxxxxxxxx@faxmaker.com where the x is the number you are sending to and faxmaker will take it from there.
Communication is a must for any organization and has to be reliable. Fax, I'm must mirror again, is here to stay as one of those forms of communication.
You can use a Windows 10 Pro machine joined to your domain as a fax server for GFI and the Brooktrout card.
For fax boards, check out e-bay. Those Brooktrout and Mainpine boards are pricey but you can find some good deals on used stuff.
JamesNT