Help with the ending!

This was not that great a movie. But did they end it weird on purpose to make up for the bad plot?

So he is in prison for life for killing a bunch of people. and yet he has access to the prison computers and is able to call her? And she has never changed her cellphone?

and how does she know it's him?

The only thing I can figure is, she kept the old cell phone active to see if he would call. and has another cell newer number which is unlisted and only her friends have the number.

So if anyone is still using the old number it would be him. right?

the movie really sucked big time and I hate it when they movie is no good, but you keep watching to the end hoping it will end well...

Can someone give me a clue, here?