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In a UMTS network, the Radio Access Bearers (RAB) configuration has a direct impact on radio resource usage. The more suited the RAB is to data flows, the more efficient it is. This paper presents an optimal RAB combination for VoIMS (VoIP with IP Multimedia Subsystem in the Core Network). The proposed RAB makes an efficient use of RObust Header Compression (ROHC) which improves the physical layer QoS. The main advantage of the new RAB combination resides in adapting the throughput for SIP (Session Initiation Protocol) signalling and improving the call setup delays for a packet switched transmission. This is achieved via an efficient algorithm of Transport Format selection in conjunction with the flexible rate matching at the physical layer.