Like it or not, faxing is still alive and kicking. No need to get a fax machine though -- instead get any of these five apps for faxing and receiving faxes on your phone.
If you don't send or receive lots of faxes, this iOS app is the one for you. There's a paid and a free version, but unless you do plenty of faxing, the free one will be what you need. With the free version you get a free fax number, although that number will be different every time you fax. You can receive 25 pages of faxes for free every month, but you can only send five pages total for free. After that you have to pay for it. The best plan is $10 per month for 500 fax pages sent and 500 fax pages received.
This one has apps for iOS and Android, and lets you do it from your desktop as well. It has plenty of nice extras, including annotating faxes and signing them. There are several levels of service, but your best bet is eFax Plus, which cost $17 a month and lets you send 150 pages or receive 150 pages. After that it's ten cents a page.
This iOS and Android app is a good choice if you don't want to pay a monthly fee, and instead prefer to pay for each fax you send and receive. You buy credits ahead of time and then apply them as you send and receive faxes.
This app is available for either iOS or Android. You won't get anything for free. Instead, you pay by the pages. For 99 cents you can fax up to five pages, and the price goes up from there. If you've got an Apple Watch you can view your faxes on it.