I think DTMF has the right idea. My single landline is connected through a fax machine and answering machine. Everything was working fine until...I installed the Ooma. I have a Hub between my modem and router (as recommended) and use Basic service - nothing fancy with cell phones or other services. 1/2 the time, my landline wouldn't ring anymore - the other half, there was a single ring that was significantly delayed and then nothing more. Also, my caller ID information disappeared from my phone.
I spent 30 minutes on the phone with Ooma without any fixes. I tried a new phone cord which happened to be longer and lost the ring entirely (not even the feeble occasional single rings). Then, I plugged an old-fashioned phone directly into the Phone jack on my Ooma and got a ring every time. My conclusion - the output voltage from the Phone jack on Ooma is inadequate to trigger a ring on modern phones or phones that are too far from the Ooma using a standard phone cable. If you used a monster cable, it might work better. As DTMF suggested, you could use a ring booster - but as far as I can tell, they're very expensive. Probably the best solution would be for Ooma to actually address this problem. This forum is filled with people having this problem and few, if any, workable solutions. Ooma wants to blame the user's home service which is silly. I went through this a few years ago with a Bose device. After 3 'refurb' replacements, Bose finally sent me hardware where the problem was actually fixed - and it has worked perfectly since. I'm not going to waste more time with Ooma trying to 'fix' it when I'm now convinced it's a deficiency in their circuitry.
My solution: I reverted my landline back to its old configuration (where it works flawlessly) and am going to buy a second phone for outgoing calls which will run through the Ooma. If Ooma didn't have such good voice quality and savings when making international calls, I would have scrapped this system immediately. As it is, I'm now going to have to live in a house where we answer one set of phones and call out on another. Two steps forward, one step back...