If you need this function right now, you can programmatically do it.
For example, monitor the alarm. If "Busy Signal Sensed" alarm returns, give ON signal to pager.send. You can use a timer to make a delay.
I don't think there is a setting to make it retry when getting the "Busy Signal Sensed" alarm. It will retry on some alarms, but seems not all. Sorry for inconvenience.
Ryan Shi
National Instruments