You don't have 8 handsets hooked to the OBi. You have three. The OBi supplies the information to the DECT base station, that you HAVE to plug into the OBi for it to function. It then supplies information to the six handsets, and it makes those handsets ring. Not do your handsets have any way to hook to RJ11 cabling...
Three handsets is not an issue. The issue is that you don't have things hooked up properly.
An OBi 100 has a port to hook into the internet, and another port to hook into a phone. Or more than one phone (via a splitter). Or the wall jack IF you disconnect the outside line feed from the wall jack so no power can come in and ruin the OBi 100.
So, if you have two phones plugged into the house wiring, and the OBi is plugged into Panasonic, then if the wired phones work you clearly still have POTs service.
So something is not being explained properly as far as your hookup, or you two wired phones aren't hooked to anything when plugged into the wall.
Please explain things a bit clearer.