cell phone question ?

well, i have a long distance relationship with a girl... i recently bought a cell phone and the day i got my phone, i received a voice to text message from her, but the message was to another guy. then a month later it happened again. when i asked her about this, she absolutely will not admit to it. i even said if she was talking to a friend, it's ok. but she still will not budge. the messages were nothing very serious, something to the effect like- how come you don't call me... and the other said just calling to say good night... she says that maybe there was some kind of mistake with the cell phones or a mistake with the communication. i don't know what to do. has anyone had anything like this happen to them before????