A couple of items:
Why someone listed Phone Amego as an alternative to this App I do not understand as I could not find any documentation that Phone Amego directly supports faxing, which is what iFax does.
That being said doing faxing via the internet defeats the primary reason that financial institutions specify faxing for critical transactions as using a real fax machines for the transmission of information insures the safety and security of the transmission. When the option of sending the information via the internet to a fax server, as it is when adding this path to the front end of the transmission (no fax machine or fax modem), it then opens up the opportunity for the contents of the fax to be hacked or compromised in another fashion, thereby defeating one of the primary purpose of faxing. The risk could be reduced if the fax app on the users computer seriously encrypted the document before sending to the fax server, which would then decrepit it before actually faxing it, but I have never seen this documented as part of any of a fax application.. Based on the available informationI have so far, using a public fax server, in my opinion seriously compromises the security and confidently of the document being transmitted, which in turn defeats one of the primary reasons for faxing, given todays technology, in the first place.
How unfortunate that Apple has chosen to eliminate the opportunity to incorporate fax modem support in the later versions of their operating systems. This is one arena that I will give high marks to Microsoft as Windows 10 still supports fax modems, which is one significant reason I still choose to run Windows on my Mac in emulation mode (Virtual Box, VMWare Fusion, Parallels).