For phone jack, which terminals are used by modem?
You need to connect to the center two wires in RJ-11. Since RJ-11 is usually 6 pins, these are pins 3 and 4. If yours has only 4 pins, these should be pins 2 and 3. Leave the other pins unconnected. 

Now, with regards to the colors (and correct polarity), this is country-dependent. Check this chart for reference. But, you can also try one way, and if it doesn't work, switch the wires.