You need to explain better.
What model OBi do you have?
How exactly do you have the regular phone line and the OBi line hooked to your phone?
In short, a home usually has a phone wire with 4 conductors in it. Red and Green are usually the ones used for the phone line. If you hook up a two-line phone, whether corded or not corded, then you need to make use of the other pair of wires in the phone line AND have a 4-line phone cord connecting the phone to the wall, and possibly connect the other two wires in the wall to the jack since many times they aren't even connected.
On the other end, you need to connect the POTS line to one set of wires and the OBi to the other set of wires, likely by bringing the house wire into a junction box and splitting off the two pairs into separate jacks, then running one cord to the OBi and another to the regular phone line.
Or, if you want two lines running through the OBi, you need the OBi 202. If you're going to use a 2-line phone, instead of two 1-line phones, you'll need to run a phone cord from each of the 202's phone ports to a splitter which brings them back into one 4 line connector which then runs to your 2-line phone.
Hope that makes sense.