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 (1, 2, 3, 10, 11, 12) connected to the SIP registrar server via a communication network, wherein each client of the number of clients (1, 2, 3, 10, 11, 12) is granted a predetermined number of register requests in a predetermined first time period, each register request corresponding to a slot, the method comprising the steps of: - lending, in the first time period, at least one slot not used by a first client of the number of clients to a second client of the number of clients (1, 2, 3, 10, 11, 12); - reclaiming, in a second time period (T1+x), the at least one slot lent to the second client of the number of clients (1, 2, 3, 10, 11, 12), wherein the step of reclaiming comprises - making the second client function as a proxy for the other clients (1, 3, 10, 11, 12) of the number of clients (1, 2, 3, 10, 11, 12) according to a third party registration process, - responding, by the SIP registrar server, in the second time period (T1+x), to the register requests of the number of clients (1, 2, 3, 10, 11, 12) with a 200OK message comprising the IP address of the second client, - responding, by the SIP registrar server, to the register request of the second client with a 200OK message comprising the IP address of the SIP registrar server, and - blacklisting the second client, if the second client does not serve the third party registrations of the other clients of the number of clients (1, 2, 3, 10, 11, 12), and wherein the step of blacklisting comprises excluding the second client from borrowing slots for sending register requests. wherein the SIP registrar server is adapted to receive register requests from a number of clients (1, 2, 3, 10, 11, 12) connected to the SIP registrar server via a communication network, wherein each client of the number of clients (1, 2, 3, 10, 11, 12) is granted a predetermined number of register requests in a predetermined first time period, each register request corresponding to a slot, the method comprising the steps of: - lending, in the first time period, at least one slot not used by a first client of the number of clients to a second client of the number of clients (1, 2, 3, 10, 11, 12); - reclaiming, in a second time period (T1+x), the at least one slot lent to the second client of the number of clients (1, 2, 3, 10, 11, 12), wherein the step of reclaiming comprises - making the second client function as a proxy for the other clients (1, 3, 10, 11, 12) of the number of clients (1, 2, 3, 10, 11, 12) according to a third party registration process, - responding, by the SIP registrar server, in the second time period (T1+x), to the register requests of the number of clients (1, 2, 3, 10, 11, 12) with a 200OK message comprising the IP address of the second client, - responding, by the SIP registrar server, to the register request of the second client with a 200OK message comprising the IP address of the SIP registrar server, and - blacklisting the second client, if the second client does not serve the third party registrations of the other clients of the number of clients (1, 2, 3, 10, 11, 12), and - lending, in the first time period, at least one slot not used by a first client of the number of clients to a second client of the number of clients (1, 2, 3, 10, 11, 12); - reclaiming, in a second time period (T1+x), the at least one slot lent to the second client of the number of clients (1, 2, 3, 10, 11, 12), wherein the step of reclaiming comprises - making the second client function as a proxy for the other clients (1, 3, 10, 11, 12) of the number of clients (1, 2, 3, 10, 11, 12) according to a third party registration process, - responding, by the SIP registrar server, in the second time period (T1+x), to the register requests of the number of clients (1, 2, 3, 10, 11, 12) with a 200OK message comprising the IP address of the second client, - responding, by the SIP registrar server, to the register request of the second client with a 200OK message comprising the IP address of the SIP registrar server, and - blacklisting the second client, if the second client does not serve the third party registrations of the other clients of the number of clients (1, 2, 3, 10, 11, 12), and - making the second client function as a proxy for the other clients (1, 3, 10, 11, 12) of the number of clients (1, 2, 3, 10, 11, 12) according to a third party registration process, - responding, by the SIP registrar server, in the second time period (T1+x), to the register requests of the number of clients (1, 2, 3, 10, 11, 12) with a 200OK message comprising the IP address of the second client, - responding, by the SIP registrar server, to the register request of the second client with a 200OK message comprising the IP address of the SIP registrar server, and - blacklisting the second client, if the second client does not serve the third party registrations of the other clients of the number of clients (1, 2, 3, 10, 11, 12), and