I would call myself a "seasoned newbie" or a "sophomore". So don't treat this info as gospel, but my sense from the info you provided is that you would need a separate router with no wifi plugged into your comcast modem, and that would give you multiple ethernet ports. Then, you could use ANY Obi device from 100 on up to plug in to one of your router slots, and sign up with Vestalink.
If my understanding is correct with regard to external routers as other threads here have talked about, you should not have a problem with comcast "blocking" your Obi phone service as it is just treated as data at that point.
Again, I stand to be corrected if something I've written is not accurate.
EDIT: I just noticed the part about your 5-port ethernet hub. That most likely takes care of the "router" that I described. All you need to do is plug your Obi into the hub, and any standard modular phone cord into the Obi.