asterisk users - Oct 2008 - "requested special control 20" ??

I'm using Teliax, and every incoming call has:

Executing [yyyxxxxxxx at teliax-in:2] Answer("IAX2/usrname-14376",
"") in
new stack
     -- Executing [yyyxxxxxx at teliax-in:3]
Dial("IAX2/usrname-14376",
"DAHDI/1,60") in new stack
     -- Called 1
     -- DAHDI/1-1 is ringing
     -- IAX2/usrname-14376 requested special control 20, passing it to 
DAHDI/1-1
     -- IAX2/usrname-14376 requested special control 20, passing it to 
DAHDI/1-1
     -- DAHDI/1-1 is ringing


It all seems to work OK, but what's "requesting special control
20" all
about?

sean