Method for managing transmission resources in a SIP-based communication system by a SIP registrar server, wherein the SIP registrar server is adapted to receive register requests from a number of clients via a communication network, and to grant a predetermined number of register requests for a predetermined first time period from each client of the number of clients (1, 2, 3, 10, 11, 12), the method comprising the steps of: - receiving a first number of register requests from a first client from the number of clients (1, 2, 3, 10, 11, 13), 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 (1, 2, 3, 10, 11, 12), 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; - 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, wherein the method further comprises a step of monitoring, within a predetermined time frame comprising at least the predetermined first time period and a predetermined number of subsequent further n time periods, the second client as to the slots used for sending register requests in the n time periods, and if the second client has unused slots, re-assigning the unused slots to a pool so as to be available for the first client, or to another client from the number of clients (1, 2, 3, 10, 11, 12) exceeding the acceptable predetermined number of register requests from the number of clients (1, 2, 3, 10, 11, 12), wherein the acceptable predetermined number of register requests is an adjustable threshold value, wherein the adjustable threshold value is reduced, if the first client or another client from the number of clients (1, 2, 3, 10, 11, 12) does not exceed the acceptable predetermined number of register requests within the predetermined time frame, and wherein if, during the step of monitoring, the second client is identified to have no unused slots, the method further comprises a step of blacklisting the second client. - receiving a first number of register requests from a first client from the number of clients (1, 2, 3, 10, 11, 13), 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 (1, 2, 3, 10, 11, 12), 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; - 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, wherein the method further comprises a step of monitoring, within a predetermined time frame comprising at least the predetermined first time period and a predetermined number of subsequent further n time periods, the second client as to the slots used for sending register requests in the n time periods, and if the second client has unused slots, re-assigning the unused slots to a pool so as to be available for the first client, or to another client from the number of clients (1, 2, 3, 10, 11, 12) exceeding the acceptable predetermined number of register requests from the number of clients (1, 2, 3, 10, 11, 12), wherein the acceptable predetermined number of register requests is an adjustable threshold value, wherein the adjustable threshold value is reduced, if the first client or another client from the number of clients (1, 2, 3, 10, 11, 12) does not exceed the acceptable predetermined number of register requests within the predetermined time frame, and wherein if, during the step of monitoring, the second client is identified to have no unused slots, the method further comprises a step of blacklisting the second client.