Changing the sim of a Nokia E71 - URGENT | Nokia E71 Fanatics | Forums

I bought a Nokia E71 as part of a 24 month contract around 13 months ago. Recently, my sister and I swapped phones, keeping our respective sim cards. The problem being that now, sometimes she cannot make calls and does not hear a dial tone, or send/receive text messages. We're both on contracts with the same network provider, and it has been said that the E71 saves data from the former sim card. Does this mean I have to reformat or master reset the phone, if so, how? Please help!