Found the reg edit that reduces the notification interval in SPB Phone Suite:

HKEY_LOCAL_MACHINE\Software\Spb Software House\Spb Phone Suite\Notifications\Delay

change Value: to (number of seconds you want in dec.)

20 seconds is the minimum interval it will do. You can enter 1-19 seconds, but it won't blink/sound/vibrate any sooner than in 20-second intervals.