Do you need a land line for a fax machine or can use the cell phone?
Actually most people are not quite accurate in response to this question. Hopefully this is more complete?!!
Answer: You normally need a landline to send a fax using a fax machine but as stated you can use an online service and if you have a smart phone you can use your mobile phone to access the service.
However, you can still buy attachments for mobile phones to enable them to send faxes direct over the airwaves but most of these mobile phones are older ones. I am actually considering buying one myself for when I am travelling. However, they seem to be out of favour and more up-to-date options are very hard to come by, especially as fax to air are so cheap in comparison. In fact after a quick search on eBay I only found 1 mobile fax machine and none would be compatible on a modern mobile phone. So I guess if you want to be retro you could use an old mobile and if you fall lucky, but otherwise I would try an app. Unless you have a 3g issue.
There are many free apps where you take a photo (scan) and the app can automatically straighten the paper. It can then be sent via email as a pdf or other attachment. Or you can send these to an online service to convert to a physical fax via a server which usually costs unless they give you a free trial.