Just by looking at that photo alone, no, we can't tell you what to do. It's the typical rat's nest of historical wiring changes.
But, if you understand basic electrical wiring, I can tell you what you can do to figure it out yourself.
The dirty block with the four big bolts is the original connection point for one or two pairs of wires coming in from the POTS telephone company (via a telephone pole or underground cable). From there, it goes to the modular jacks inside the box. the jacks are intended as a customer-accessible disconnect and wiring test point. For example, if there was a problem with the house wiring, the customer could unplug the modular jacks for line 1 and line 2, and plug a modular phone cord and phone directly in there, to see if it works.
Telephones are wired according to the diagram attached. Note the house wiring in most houses built in the 1970s or newer use 8-conductor, Cat-5 twisted pair cable, with each pair having a primary color, and a white wire with stripes of that primary color. So, line 1 is always blue and blue/white, and line 2 is always orange with white/orange stripe, etc.
Your challenge is to find and disconnect all the wires from both the old POTS telephone company, AND from the Comcast Xfinity service. If you don't do that, you might fry the OBi. So, the first thing that should be disconnected are the blue/blue+white and orange/orange+white wires off of the 4-bolt connector block. The next thing to do is to determine if you have an security alarm panel wired to the phone line. IF you do, it makes things much more confusing to trace.
There will likely be (based on the jumble of phone cable on the left side of the photo) several different runs of cable, to different parts of your house, including the one that Comcast ran to backfeed from their MTU (cable modem/router/VoIP device). The goal is to connect all of the cables together (all blue wires together all blue+white wires together, etc.). However, if you have an alarm, then the very confusing thing will be that the phone line was originally looped from that box, to the alarm panel, and then back to the box, and from there, to the phones in the house. This may involve a color change. But, from the photo, all I can see are the L1 and L2 pairs.
The end result should be:
Old POTS Telco /cut/ cable from OBi--->house wiring--->jacks
Old Comcast Telco /cut/
If you aren't comfortable tracing this out, then the best alternative is to just plug the cordless phone base into the OBi, and use cordless phones, or hire an electrician with low-voltage wiring expertise.
