I'm better at wiring than I was in configuring GV so here is an idea:
For the test, simply plug the base unit into the OBI device. Don't worry about wiring at all. Once that works, you have the 5 cordless phones working on the OBI. Rather than do any wiring changes at all, how about buying another cordless phone to add to the 5 you already have. Put that one in the kitchen in place of the current wired phone.
That way, you will need to do no wiring changes other than plugging the cordless base into the OBI.
Yes, you could use an unused pair, e.g. black-yellow to back-feed the voice from the OBI down to the kitchen phone, but why go to that work. You would need to change the kitchen phone to the black-yellow as well as the change in the office that back-feeds the new voice line via OBI.
I have used Centurylink and also a regional cable company. CL is many times more reliable than the cable. Much fewer outages and many, many times less short term stutters. When cable works, it is fantastic. Unless it is having an issue and web pages keep saying not reachable even though speed test is lightning fast. Happens too often with cable, almost never with CL.