If you go here:
You'll see that the free calculator calculates 24Kbps bandwidth required for a G729A (+overhead) voice path - obviously not doubling the value. NetIQ Vivinet Assessor's white paper "Planning for VoIP", section "CODEC Selection" document says to double the one-way value when doing capacity planning.
So, who's right?
The bandwidth rate is for one direction. So G729A for example requires ~24Kbps in each direction.
Even during periods of silence (say, for example when one person in the conversation is just listening), VoIP by default actually sends a continuous stream of packets at the prescribed rate. BUT, you can turn that off by using what's called a silence suppression algorithm (which is very commonly used; more often than not).
The encoders can detect these periods of silence and simply not forward any packets that contain nothing but silence. This of course reduces the overall number of packets in a given call.
A typical phone call might have silence 50% of the time when taking both directions into account, but of course it could vary. As a result, actual bandwidth utilization could be anywhere from 24Kbps to 48Kbps depending on whether silence suppression is used, and the percentage of actual silence time in each direction.
The AppManager VoIPQuality module and Vivinet Assessor both have the ability to turn silence suppression on/off, and to control the percentage of silence.