For VoIP with G.711, you generally need 64kbps + x in either direction (64+x (up) AND 64+x (down)). That’s because you have to build up 2 separate voice paths (one from subscriber A to subscriber B and another one in the opposite direction).
By the way: if you assume a usual ip network, “x” (in the above calculation) is NOT 16 (add up header sizes of rtp, udp, ip + (eventually) ethernet)! Or did you mean a plus of 16 kbps for signalling? usually you don’t have to count the signalling bandwidth for voip because signalling is done in a totally differenz way than it’s done in isdn. there’s no need for a certain signalling bandwidth. have you heard about the session initation protocol (SIP)? That’s just one possibility to do signalling in voip.
ISDN uses ONE voice path (one so called B-channel) for bidirectional talking, which means that the voice data from A and B are added up and transported in an overlaid manner in the same data stream. That’s a main difference between voip and isdn. in voip, you open up two unidirectional “voice channels” (one from A to B and one vice versa), in isdn you just have ONE channel for both directions.