If it's not set for "one network outage only" it will forward all calls. When I travel abroad, I often turn the box off completely, along with a whole mess of other items while traveling. Works fine.

you CAN also use the iphone/android app now to make, receive calls. Works quite well actually, calls come directly to the device, regardless of connection type. Helps segregate calls - AND calling for ooma users (maybe premier only) is free on the mobile app regardless of location. Sound quality is much better than skype as well.