asterisk users - Dec 2003 - dynamically enabling PRI channels

Is there a simple way to enable/disable on-the-fly a pri channel?

I want to control the number of incoming lines based on the number of agents I 
have.

I took a look at libpri and zaptel and found some hints about a
"SERVICE"
message within the pri protocol, but coudn't find out much more about it.

Best,

PHM