From your description and the fact that you had DSL at one time, it would appear that your DSL splitter is built into your phone company interface box. With that in mind, there are usually two rj-11 jacks with two short cables plugged into them. For a simple test, you should be able to unplug the short cable from the "voice" plug and plug it into the "Phone" port on the Obi. The jack will be left empty. Since the phone company "box" is usually on the outside of the house you won't likely want to leave the Obi outside but this should work as a test and provide your phones in the house with dial tone from the Obi. The only problem you might have is if there are too many phones [5] plugged into the line in the house it could load the Obi too much. As the other person noted, just make 100% sure that your line coming from the phone company is dead if you aren't familiar with the wiring and / or use of a voltmeter.
*Side note* - If you had telephone service from Comcast, the DSL "box" may have already been disconnected from the house telephone wiring by Comcast and you will not be able to connect the Obi there.