If you make many landline calls, you can set up the OBi to automatically select the line based on the number dialed. For example, if the landline has unlimited local calling, you might choose to send all calls in your local area code that way. Or, if it's a pseudo-landline, e.g. digital voice from your cable company, with unlimited domestic calling but high international rates, you could send all domestic calls via cable voice.
Whatever automatic behavior you set up can be overridden as desired: **1 to force a call to use SP1, **8 to force use of the landline.