Why does the plant imitate a cell phone? Because it's a sound that humans will instinctively approach. (They also learned the sound from one of their previous victims, which Amy stumbles into when searching for the source of the "ringing").
There's a good reason why the Mayans were trying to contain the vines-but Amy escaped the quarantine. Guess what—now the Mayans have to find and destroy her body (and the associated growth) or the entire ecosystem is likely screwed.