Wired is the cheapest solution. As telephone requires duplex you will need two twisted pairs, on for RX and one for TX (telephone cable will do). It is possible to multiplex RX and TX on one twisted pair but that would only add to your cost. Drive TX differentially and receive RX differentially. This is to block interfering signals that appear on your cables as common mode signal.
A good solution can be to use a ADSL driver/receiver pair, like the MAX4363. Perhaps the chip is a little overspecified to this purpose, but it's good and cheap, so what do you need more? If the ADSL driver/receiver gain is not enough, you can add some gain stages on the main board.
It's jsut an idea!