[CyberFonic]

The technology problem is at the fax machine end.

Fax machines have embedded modems which decode and respond with a complex set of tones to transmit and receive data that makes up the facsimile image (yup, it's all images to the fax and that's why they still use them).

Smartphones are all digital so you need a gateway to convert digital signals to suitable analog tones to satisfy the fax machine's modem. Conceptually you could implement the gateway functionality as an app because we can still transmit and receive tones, i.e. voice calls. Quantisation and delays might still introduce errors, but the fax protocol does have some basic error checking and re-transmit capabilities.