In a service level interworking unit, a service center (SC) address or a set of SC addresses dedicated for the service level interworking is used whenever an instant message (IM) is interworked to a short message (SM). In other words, if a message is initially sent as an instant message (IM) by user equipment (UE) and is interworked to a short message (SM) by the interworking function, the interworked short message (SM) is sent to a dedicated service center (SC) address. For originating messages that are not interworked, different service center address or addresses are used. A terminating short message (SM) received at the interworking unit is interworked to an instant message (IM) and the interworked instant message (IM) is delivered to the user equipment (UE), if a service center address in the terminating short message (SM) is one of the dedicated service center addresses. If the a service center address in the terminating short message (SM) is not one of the dedicated service center addresses, the terminating short message (SM) is delivered to the user equipment (UE) without a service level interworking.