for E1/T1 trunk lines you are right. I just figured it out from the ISDN BRA subscriber line (Uk0) point of view. There you’ve got 2 B-Channels (which in fact are both bidirectionally used) with 64kbps each plus one 16kbps D-Channel for signaling.
That kind of bidirectional voice transmission over one 64 kbps channel is realized by the use of echo cancellation (ITU-T G.165/G.168 I think). Unfortunately I don’t know the english expression for that kind of technique that is used to transmit the voice stream bidirectional A B on just one 64 kbps B-channel (in german it’s called Gleichlageverfahren). But it really works that way, otherwise it would not be possible to have two independent calls on just one ISDN BRA subscriber line (which in fact is possible).
I just didn’t think about E1/T1 because VOIP Beginner wrote about (64+16) kbps. I think with the “+16” he means the bandwidth of the D-Channel that’s used for signaling on a BRA subscriber line.