Cisco AS5300 gateway will generate a 20-byte voice sample every 20 ms or 50 pps. Adding the 12-byte RTP header, eight-byte Unreliable Datagram Protocol (UDP) header, and 20-byte IP header to the packet, the IP datagram will total 60 bytes. With 6 bytes of link-layer overhead, this generates a 66-byte packet every 20 msec (without voice activity detection [VAD]). With

VAD enabled, this translates to:

66 bytes/packet * 8 bits/byte * 1/2 * 50 pps = 13.2 kbps per call or DS0

for more info

http://www.cisco.com/en/US/tech/tk1330/technologies_design_guide_chapter09186a008075aaf9.html