Where was Rose calling from?

When Rose first calls she is not in her boyfriend's apartment. We know this because she says she walked by and didn't see him there. However, she later moves into the apartment and continues her phone calls. At this point is she not calling the same number? Her number? You can't do that or is it possible across time barriers? Or was she living in another place and using the apartment which may explain why Mary would have rented an apartment where she had been abducted to and severely burned in.