Many standard analog telephones (desk/wall/cordless) can get the time of day automatically from the caller ID service. When a call comes in, it has a timestamp, and so the phone sets its clock to that time. So, you need to receive at least one inbound call for that to happen. Of course, this assumes that the telephone in question actually has that feature. Many older phones don't (obviously not, if they don't have caller ID displays). If your phone has a display AND caller ID, it should set the clock, as long as that setting is enabled on the phone.
This assumes that your OBi is actually online and you configured it with the correct time zone.