First of all: Be careful not to connect telephone lines directly to any part of your computer system. Except the phone modem port obviously.
Reason: The telephone company puts 60V DC (or -60V) on one of the incoming wires. It can ruin unprotected computer components instantly (regardless of the fact that the 60V is low impedance).
An ISDN line needs a special box between the incoming phone lines and the computer equipment.
Have you located the box? In the basement? Garage? Etc. Have you looked for an ordinary phone outlet near the box so you can plug an analog phone into it? If there's no outlet then it will take some knowhow to find the two incoming terminals that provide you with phone service.
Or do you want to adapt telephone signals to your computer? Do you want to transmit voice in any way, shape or form ? You'll need to digitize it with an analog-to-digital converter. That's only the first step. You may want to find a commercial device to achieve what you want.
Or do you want to transmit tones produced when you press your phone keypad? This will require that you locate the analog lines to your equipment. The instructions probably don't say anything about it. You'll need detailed knowhow. A commercial device may be needed.