Call Files and multiple lines

Hello everybody,

First, i will describe my current asterisk installation then i will explain my problem.

I use Asterisk v1.6.2.13 on a Debian. My goal is to make call from calls files, using a group of lines. I have 50 lines with 2 channels each. Moreover, i use AGI Script with the asterisk-java library.

In my extensions.conf, i have the following lines :

exten => 50000, 1, Agi(agi://localhost/alarm.agi)

And here is a sample call file :

Channel: SIP/testline/33102030405
Context: alarm-context
Extension: 50000
MaxRetries: 2
RetryTime: 5

Each line have only two channels. My problem is : How can I do for that call files use automatically phones lines not busy ?

When a call file will be put in the /var/spool/asterisk/outgoing/ folder and will be run, i want that it use a phone line not busy.

Can you help me ?

Best regards,