Hi,
Not long ago I was asked at work for a simple digital monitoring system for a one off installation, using an existing UHF FM comms system for voice.
The owner wanted minimal intrusion into the existing system, it was to be used at night when voice comms was not needed.
Found this library, it uses;
SoftModem is a wired, low-cost and platform independent solution for communication between an Arduino and mobile phones. It uses the phone's audio jack and Bell 202 modem-like FSK encoding with up to 1225 bit/s.
It is designed for phone application, so I coded a PTT line and some pairing protocol to check connection.
I used the lowest baud rate;
#define SOFT_MODEM_BAUD_RATE (126)
#define SOFT_MODEM_LOW_FREQ (882)
#define SOFT_MODEM_HIGH_FREQ (1764)
Using speaker wire will work, that's how I started my DEVELOPMENT IN STAGES of the project.
I left the transceivers out of the comms loop first to prove hardware and library performance.
Tom.... 
PS, many would have seen this picture, its the prototype that ended up spending a week in the field till the PCB was assembled and tested.
