G.729 users 8Kbps theoritically it should be 155Mbps/8Kbps, but its not the case since there are other factors as well which consumes the available bandwidth for transporting the voice traffic.
also you can make use of compression techniques to push more calls simultaneously within the given bandwidth
i would suggest to try this link which i find quite interesting...
STM-1 connection have only 63E1 (126Mbps) which carry your data and 1 E1 = 32 timeslots (i think only 31 carry voice) .
Rest (155-126) is overhead now you can see how many channels you can get for 711 or 729.