modem -> Ooma -> Computer
without a router.
You have probably not configured the Ooma to use it's built-in MAC address so it is using a clone of the computer's MAC address. When you turn the computer off Ooma switches to use it's own MAC address at which point the modem blocks the connection because a modem, in a residential environment, allows only one IP address to be issued.
Go to the Internet page, click the Use built-in button, and then click the Update device button.
Turn the modem, Ooma, and the computer off.
Power the modem and wait for it to sync with your ISP.
Turn on Ooma and wait for it to come online.
Turn on the computer.
You should now be able to make phone calls independent of the computer's state.
Telo with 2 Handsets, a Linx, and a Safety Phone
Telo2 with 2 Handsets and a Linx