Call your PSTN party from your Obi, then add the other Obi. You will dial using **9 to get the second Obi in the conversation
3-Way Calling
3-Way Calling allows you to talk to two parties at the same time with everyone on a telephone at a different location. To use 3-Way Calling, when you are in a call with another party and want to add a second to the conversation, press the "flash" button or depress and release the switch hook on your phone. You will be presented with a second dial tone and the first party will be placed on hold. Dial the second party. When they answer, you will be able to inform them that you intend to connect them with the first party (now on hold) and have a conference. At this point press the "flash" button or depress and release the switch hook on your phone. This will connect the first party, the second party and yourself. You can all continue to talk together.