Well kind of sort off.
Yes, each call takes two voice paths.One path to transmit and one to receive from the data world.
But when you get a T1 from your data provider, you are getting two 1.5 mb paths too.
That means that you are able to transmit 1.5 mb and receive 1.5 mb at the same time.
Your calculation of posible 64 channels of VOIp over a T1 is correct.
That is kind of conservative , but better be safe than sorry. It comes out to around 24 kb per call.