Incoming and outgoing channels

Hi all,

I am new to asterisk, but I’ve recently configured an IVR that accept incoming calls from customers and redirect them to an external number(the customer service in their area).
My problem is that I can’t really understand a simple principle on how channels are being managed.
So I’ll explain my ambient:

I have a four span Digium TE420B card with 2 E1 PRI attached(in two different spans).
Now, in total I’m having 60 channels available, but what I can’t understand is if I should use all of them in a group in dahdi.conf, so asterisk can manage all of them separating incoming and outgoing channels, or if I have to define two separate gorups(one for managing 30 incoming and one for 30 outgoing calls) and defining the outgoing group in the dialplan when I use the Dial() command.
And if so, how should asterisk know how to keep incoming calls in group one?

I know it’s a simple question, but I can’t find any information.
If you need my configuration files I’ll post them, just ask.

Hope someone will help me,
Thanks in advance.