A bandwidth management system and a method adapted for the system. The system determines when the sum of the required bandwidth of a new VOIP call and previous VOIP calls from a client device is greater than the maximum sustained rate, transmits a SIP invite to a called party of the new VOIP call to execute a SIP negotiation via a base station and monitors the new VOIP call and the VOIP calls whose SIP negotiations are not successful. When one of the VOIP calls of awaiting SIP negotiations is successful, the system calculates an actual used bandwidth of the client device, and when the actual used bandwidth is equal to or less than the maximum sustained rate, the system establishes a call link for the one VOIP call and monitoring the other VOIP calls of awaiting SIP negotiations.