If you mean the warning tone you hear in the vehicle, it only activates if there is an object close enough to the bumper as you are backing. I did some testing after I bought my 2015 TC Wagon and how close depends on the size of the object. A large flat object gets detected farther away, with the tone going continuous about a foot away as mentioned in the manual on pages 128-129. But even with just a single piece of 1/2" conduit sticking up out of the ground in an open flat yard, what I thought would be hardest to detect and thus most likely to hit, the tone went continuous at about 4 inches.
But if you mean a continuous beeping heard outside the vehicle the entire time the vehicle is backing as a warning for others, that seems to be what this add on accessory is for:
Backup Alarm by ECCO
Part No: VAC3Z-14N137-A SRP: $105.00 http://accessories.ford.com/back-up-alarm.html
But it's just a guess because the description is rather sparse.