Eseries / Communicators :: Changing The Sim Card E71

Last year I bought an E71 as part of a 24 month contract. Recently, my sister and I swapped phones and kept our respective sim cards (which are on the same network provider), my former phone accepts the sim card and she is able to send and recieve texts, and phone calls. The problem being that when she tries to make calls (from my former phone) there sometimes is no dial tone. I was told that the phone had saved data from my sim card and needed to be reformatted, is this the case?