[asterisk-users] distribute free call minutes over different channels

[Eckhard Jokisch]

Hi,
I have 4 ISDN channels (2 lines) and each line may do calls of up to 360 
minutes/month for free. 
As I understand asterisk will pick the first available line so the probability 
is big that the other lines will not use their free minutes and the firs line 
will exceed the free minutes.
How can I configure asterisk in a way that it looks up in the CDR which ISDN 
line has lest calling time in the present month and chosse this?

Kind regards
Eckhard