On 3640, you can dedicate the whole T1/E1 line for dialin or dialout for Async calls over PRI/CAS line but not the certain channels from those lines..So i don't think that there is a way to to reserve certain number of channels for dialin/out..
But you can use dialer-map statement with fixed/static IP for those 5 dialout sessions and still can negotiate ip for dialin sessions...Thanks..Tejal