I understand this is a issue. Cisco IOS router's can't designate or reserve some number of channels for dialin or dialout only. However you can designate the whole controller for dialout using "modem dialout contoller " under the line config.
Here is the link for more on that
http://www.cisco.com/en/US/tech/tk801/tk36/technologies_configuration_example09186a0080093c1b.shtml
I am afraid that there is no feature available to select the channel for dialout from the router. but is you have ISDN signalling, the closest you can get is to use the comamnd "isdn bchan-number-order " command for ascending or descending for dialout only.
Router(config-if)#isdn bchan-number-order ?
ascending B-chan selection starting from B1
descending B-chan selection starting from B23/32