Firstly: DO NOT CONNECT THE PIC DIRECTLY TO THE PHONE LINE!
Not only will you upset the telephone company but you will almost certainly destroy the PIC as well. Telephone lines are potentially very dangerous, thats why the line wires are always insulated and unreachable. They can easily have > 100V on them and none of the wires (there are usually two but can be more) are ground.
To sense ringing voltage you have to isolate the AC ringing supply from the background DC on the line. For that you need a safety rated capacitor, a current limiter, a rectifier and an opto-isolator. You probably need a voltage deadband circuit to prevent random noise and static on the line looking like a real ringing signal too.
To seize the line (answer the call) you have to connect a constant current load across the wires. For this you need a relay and a constant current circuit. Typically this will be a small power transistor with fixed base voltage and a control resistor in it's emitter.
Then you have to modulate something on the line to send your 400 bytes. As phone lines are typically rated to carry low frequencies between about 300Hz and 3.5KHz, you have to decide if you are sending the data as keyed amplitude (similar to Morse code) or keyed frequency (FSK) and design a modulator to suit. Obviously this has to be compatible with the system it talks to at the other end of the call. You then current modulate it to the line using an isolating transformer.
The PIC would be responsible for monitoring the ringing signal, operating the line relay, preparing the data for the modulator and hand up when the data has ended. It also need it's own power supply which must be isolated from the phone line.
So it isn't as easy as it sounds. Bear in mind that a commercial modem does all of the above and usually just sends and receives data from the PIC UART pins. That's why Klaus is saying "don't re-invent the wheel" particularly when it would be expensive and complicated to do so.
Brian.