This is from my own experience today: I wanted to use my landline to call (sounds better; maybe psychological though) instead of talking to my n800, so on GC mobile Web site I changed my "mobile" number from Gizmo to landline. When I initiated the call from GC, only my landline ringed, not n800. So it did work, I think. After the call, I changed the mobile number to Gizmo again (this may not be necessary of course).