having problems with my nokia 2730 mobile phone?

i have a nkoia 2730 classic and its just started to not let me phonre anybody it keeps saying connection error or just comes up connection not possible all i have done today is change the time on it i have tried to restore it back to factory settings but its asking for a security code but ive never used one on it ive tried 0000 but no good ive also removed the battery which has done no good also other people cannot phone me can you help thanks