I originally thought that fax software would be the cheapest solution for my business, until I realized that most companies charge per page, once you start faxing over a certain number of pages each month.
After my research, I realized that I could purchase a multi-function machine (copier/scanner/faxer/printer), and fax files directly from my computer, without printing them.
I went with Brother's MFC-465CN machine. I know you said you want to get rid of your landline, but when I compared the costs of my telephone versus the fax software that was out there, I discovered that the option I went with was the most cost effective, at least for me.

Hope this helps.

Joe