Almost no one use fax anymore so what do you do when someone demand that you send them documents by fax? Luckily there are apps for sending faxes.
Since I use Scanbot for scanning documents, and it has fax capabilities, I would use that as my firsthand choice. Scanbot is a free app but you have to pay for sending faxes. That is reasonable since they have to pay for sending faxes and maintaining the fax service.
Open settings in Scanbot. There you can buy fax credits. You have the option to sync your credits to your iCloud account. That way you can use them on all your iOS devices. Scanbot is a universal app.