I would guess that the vets examined the crocodile, and through the aid of a stethoscope, were able to hear the ringing phone.
Either that, or she has a very loud and obnoxious ringtone!
How was the woman able to convince the aquarium officials to take the croc out and examine him on her say so? That seems like it would be quite a job, and how was she able to identify which croc swallowed her phone? Don't they all look pretty much alike? How long before the cell phone battery needed charging? So many questions..........