Some telephone line cords have two wires (what's needed for one line) and some have four (the other pair is used for a second line or a special purpose). If you look at either plug end, you can tell how many wires are there.
The OBi202 supports both lines connected through the Phone 1 port, using a four-wire cord. This requires that the phone also support this. If so, the jack on the phone is usually labeled "L1/L2" or similar.
The OP should try using a four-wire cord connecting OBi Phone 1 to the phone's L1 jack, with nothing plugged in to the Phone 2 port. It's possible that both lines will work this way, with just the one cord.
If not, the OP should use two cords, with the Phone 1 port cord having only two wires, and then both lines should work fine.