Preventing new calls without disconnecting current

I would like the ability to prevent new calls from establishing through my Net-Net SD, without impacting current active calls. I would use this to allow existing calls to complete and disconnect, then I could perform maintenance/reboots, etc without risking impacting users (I have a secondary HA pair that would take the new calls).

 

Is there a recommended way to do this?

Or is the preference to just let HA fail the calls back-n-forth whilst upgrades/reboots occur?

 

Thanks in advance,

-Martin