A computer apparatus (e.g. a server) and a method for managing transmission resources in a SIP-based communication system can include use of a SIP registrar server that is adapted to receive register requests from a number of clients connected to the SIP registrar server via a communication network. Each client can be granted a predetermined number of register requests in a predetermined first time period where each register request corresponds to a slot. In the first time period, at least one slot not used by a first client can be lent to a second client. In a second time period, the at least one slot lent to the second client can be reclaimed by the first client or another client. The reclaiming of the lent slot(s) can include making the second client function as a proxy for other clients according to a third party registration process.