When you make a call through the OBi, it waits until you're done dialing a number in order decide what to do with the call. If the call is to be sent to the LINE Port, then the OBi has to take the LINE Port off-hook, wait 0.5 seconds for a dialtone, and then dial the number through the LINE Port. It takes 0.4 seconds per digit, so an 11-digit number would take a total of about 4.9 seconds to dial.
When the OBi is turned off, the PHONE Port is connected directly to the LINE Port, so the OBi is totally out of the picture.
If you'd like to bypass the OBi and dial directly out the LINE Port, press #, wait for a dialtone, and then dial the desired number.
Could the noise you occasionally hear be a Call Waiting indication of an incoming call? If it is, pressing the Flash button on the phone should place the current call on hold and answer the incoming call.