03
All of former East Germany and Berlin
East Germany was using +37 before the reunification. After reunification, East Germany was merged into the existing (West) German numbering plan. Since all areas except 03 were already used, all of former East Germany needed to be merged into 03, causing numbers and area codes in the 03 area to be longer than those in the rest of Germany: Many area codes in the 03 area are 5-digit while the maximum in the rest of Germany is 4 digits
Read more about this topic: List Of Dialling Codes In Germany