Do you have something similar to this in rpt.conf?

idrecording=|iW3SBA
idtime=600000                          ; id interval time (in ms) (optional)
politeid=3000                            ; time in milliseconds before ID timer

Also, in "duplex=" modes 0 and 1,  if the transmitter is idle, it will not turn on the transmitter to send an ID.
Even if the timer is due.
It will send the ID if the transmitter is already up though.

Take a look at :
http://ohnosec.org/drupal/node/128
http://ohnosec.org/drupal/node/129
http://ohnosec.org/drupal/node/87

And on http://ohnosec.org/drupal/node/150 (undocumented features) I find the following

rptinactmacro  Macro to execute when inactivity timer expires 
rptinacttime   Inactivity timer time in seconds  (0 seconds disables feature)

But I've never tried them to see if they work.