OBiTALK ATAs always generate a dial tone (internally, in the box), even if they are disconnected from the network.
Try unplugging both the Ethernet cable and the power cable. Wait minute, then plug back in just the power cable and wait for the device to reboot. It will be normal for one of the LEDs to blink on and off, because it's trying to get an IP address, and it can't. Pick up the phone. If there is no dialtone, then either the OBi or its power supply are defective. If no dialtone, check to see if you have a spare 12V 1A or 2A power supply with the same connector, and swap them out. These power supplies are commonly used by routers, switches and other home network gear.