Thanks guys, just a quick update-- after 3 hours with the phone service guy, I think I'm all set to try this out.
The guy came, was able to find exactly what MichiganTelephone mentioned as an alternative:
"Alternately, it may be possible to find an unused pair in existing phone wiring that could be used to make the connection, as long as you can ascertain that it's not connected to any lines coming from the telephone company, and you can ascertain that it never will be..."
He found two extra unused pairs in the existing phone wiring-- so I asked him to install two extra phone lines running from the upstairs office (where the modem is) all the way down to the basement (where the KX-TA624 main unit is)-- took a bit of time since there was a bridge in the attic, but with his fancy toner and equipment he was able to find the lines in the bridge and follow them all the way back down to the basement. Great bootstrapping if you ask me!
Anyways I now tested both of these new lines out with an Ooma Core that I've been running upstairs in the office as a fax line-- it used to stay completely separate from the main house phone wiring-- simply modem to router to Ooma core to fax machine and it's worked perfectly. I now sent it from modem to router to Ooma core to one of the new phone jacks that the guy installed, it runs all the way down to the basement and from the new jack downstairs straight into the CO3 port of the KX-TA624. It works perfectly and now I have an Ooma line as CO3 throughout the house and all 17 of its extensions!
I just ordered the Obi110 and will use that for the second new line and install that into the CO2 or CO4 port of the KX-TA624. It should work, so I'm excited. At that point, I'll probably switch Ooma back to just using it as a dedicated fax line (since Ooma works with fax, and I don't think the Obi does, no?), and use Obi as a voice line throughout the house, or I'll use both since I have two lines to play with (and just split the Ooma so it goes to fax and to the KX-TA624)...
Anyways I'll keep you posted once my Obi110 shows up in the mail, but for now things are looking good.
Last question-- has anyone put an Ooma Core phone line into the FX port of the Obi110? Is there any possible reason one would want to do this?
thanks again!
-A