Is there any chance you have the telephone connected to the LINE Port and the PSTN line connected to the PHONE Port?
With power removed from the OBi, a fail-safe relay connects the PHONE Port directly to the LINE Port with nothing else in between and your telephone receives PSTN dialtone.
With power applied to the OBi, the PHONE and LINE Ports are connected to the OBi's hardware. Dialtone should be supplied by the OBi at this point.
An OBi should always provide dialtone from its PHONE Port when it's powered up, even if the OBi is totally unconfigured. If you cannot get dialone from the OBi when you pick up the telephone connected to the PHONE Port, you may have a defective OBi.