A method for managing transmission resources in a SIP-based communication system by a SIP registrar server can include receiving a first number of register requests from a first client from the number of clients, each register request corresponding to one slot in the predetermined first time period; receiving a second number of register requests from a second client of the number of clients, each register request corresponding to one slot in the predetermined first time period, wherein the second number of register requests exceeds the acceptable predetermined number of register requests for the second client, and assigning a number of slots not used by the first client within the predetermined first time period to the second client for sending the register requests which exceed the acceptable predetermined number of register requests for the second client. A system for implementation of the method and an apparatus can be utilized in embodiments of the method and the system.