The calc is a very good tool, but your assumption is correct, you need PVDM channel for each voice port in low complexity codec (g711), if you are going to use medium or high complexity codecs (g729) then 2PVDMs are used for each call, so make sure you know what time of calls you will have. For example if a call arrives at site A and connects to phone at siteA with G711 codec (based on region configuration) but then phoneA transfers this call to siteB phoneB and region between siteA and B is set to use G729, the GW will re-negotiate the codec and G729 will be used taking additional PVDM channel.
HTH,
Chris
