Please realize that recall (coming back when called) is the least of your worries with outdoor free flight. Cockatiels are small prey animals, and they're protected outside by staying in large flocks. Without a group of buddies to fly with, she'd be in constant danger. Hawks and owls inhabit even the most urban areas, and birds you don't necessarily think of as a threat... seagulls, crows, mockingbirds etc., will chase strange birds for miles. Without buddies to help keep a lookout, she's a sitting duck.
Dogs and cats are always a risk, too, as are cars and bodies of water, as birds that are spooked often fly until they're exhausted and then land in the worst possible places when they simply can't go any further.
Heck, the air currents themselves are dangerous to a free flying bird. My quaker is a very skillful flier at home, but flying indoors is like swimming in a pool... no matter how good you are, it won't prepare you for swimming in the ocean. Even a light breeze, to a small bird, is essentially a nasty riptide that can whisk them away in a moment. It may not be that they don't want to come back to you, sometimes they just can't.
I've thought about free flying, and if I had a large colorful bird like a macaw, one that is visible (and therefore findable) from a long way off and big enough to make predatory birds think twice, I might consider it (and even then, I'd seek out other free fliers to provide my bird with the safety of numbers), but my little tiel? All by himself? No way.
