Powering off the OBi200 will also disable the Home phone. Is this what you want?
You can set DND by dialing a star code before you leave for work and the same to turn off DND when you return. The star code will have to be modified to turn on DND for only the Work phone.
Assuming Work phone is defined on SP1:
*78, Do Not Disturb, set(sp1($Dnd),1)
*79, Disable DND, set(sp1($Dnd),0)