The site that broke the story admits it was "acquired from a third party". They claim this is all from her, but not once is there a mention of her talking to them, merely the information presented by the third party. At that point, it's purely hearsay.
All it takes is a
PR person. It was probably a company provided phone that when she left, she turned back over. It's much easier than you think to access things like voicemails in general, but when you put someone in the middle who literally serves to make communication easier, you've got a pretty solid match.
At the end of the day, Sterger hasn't said a damn thing. There isn't a single reputable story the claims she has. All that's been reported is that she received texts and voicemails, and that
someone got a hold of them. If she was indeed the one coming forward, why would she hesitate to talk to the NFL and expedite this "getting rich" process?