sorry, I misunderstood, I assumed you were trying to send/receive over the telephone network rather than just on the telephone line wires.
What you need to do is this:
The phone wiring MUST be disconnected from external lines first!
The wires are not screened so directly carrying low level signals will result in lots of hum and interference. Depending on the volume you need at the outputs, you may be able to connect the SPEAKER output from the sound card (higher level & lower impedance than the headphone output) directly to one pair of wires and tap loudspeakers or whatever you use off the same wires elsewhere. Essentially, using the phone line as an extension speaker cable.
or
You can eliminate the interference by driving the line and receiving from it through transformers. This method will reduce the interference so it is low enough to drive from the headphone socket and be amplified at the receiving end of the connection. The idea is that you use a center tapped transformer and send equal but opposite signals on two phone line wires. At the receiving end, you invert one signal and add it to the other, either with another transformer or a differential input amplifier. As the interference will be approximately equal on the wires because of their close proximity to each other, when you invert the signal on one wire and add it to the other, they cancel out. The signal that you inverted at the sound card end is also inverted but as it was already upside down, this time it adds rather than cancels so you get twice the signal level.
Brian.