The maximum bandwidth utilized for one voice call using Codec g729 is 26kb/s.
If you use g729 with rtp header compression the Overhead of 40byte header is compressed to about 4byte and the overall bandwidth per one call would then be
[ 8kb/s _ (1+4/20)]+ 2 =
[ 8 _ 1.2] + 2
9.6 +2 = 11.6kb/s
If you have VAD enabled, it can further reduce bandwidth by not producing rtp packets in time of silence. This can further the bandwidth consumed by voice
call by 35% hence the overall minimum is 65% of 11.6 KB/s
7.54kb/s.