workflow conditions check

hi , i am trying to use workflow to update my record option set value & two option set value

      if(mainphone="123")then
update:(option set value;two option set value)

     otherwise if(mainphone="456")then

update:(option set value;two option set value)

     otherwise if(mainphone =" ") then

update(clear)

this is the condition. After the first on-field change ,my option set value is not getting updated(override) for the second time.
is there any problem with the logic.how to rectify.