Dial out to PSTN

Hi all, I have 3 analogue lines, 1 of them is only for dialing out and asterisk is not going to pick up any call from that line, and the rest are assisted to throw to different context.
Here is my question, I want to know how to make the asterisk not picking up the call from specific line while letting you to use that line to dial out. And I want to let the asterisk to chose the available line for dial out.
How should I configure the asterisk?

Thank you.