By default, if the spa9000 finds that the 1st PSTN line of spa400 is busy, it will try the next PSTN line. Mapping each specific PSTN line to line key on the phone isn't supported.
You can map a speed dial to a Line Key, use the extended function parameter (located in Phone/Line Key) and do something like, fnc=sp;ext=400@$PROXY so that matching line key can be used to speed dial to in this case, ext 400. Further details are in the admin guide.