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.
The movie seems to suggest that the vine can only get into the bloodstream through a cut. A big deal is made about Stacy accidentally cutting Jeff's hand - he's now vulnerable to getting infected. Notice how Eric never gets cut and he is never shown to have it growing inside him. Additionally I'm sure there are chemicals capable of destroying the vine once Amy gets back to civilisation. Of course this depends on what ending you take as canon - if you accept the one where Amy has the vine growing out of her eye then yes the ecosystem is screwed.