SIP signaling traffic is transmitted over a default bearer.
When the first voice traffic packet is detected by P-GW, the P-GW initiates a dedicated bearer activation procedure according to the local rules. The dedicated bearer is established only when there is no dedicated bearer already established for the voice traffic.
The P-GW uses this function for VoIP services so that SIP signaling traffic is transmitted over the default bearer and the voice traffic is transmitted over the new dedicated bearer.
