In the configuration tool go to Gateway configuration > GSM outgoing groups and Set Mode of switching SIM card to Random. Then go to Time limits and set time interval Time limitation from 03:59 to 04:01. This time interval has to be set for all inserted SIM cards (SIM 1 - SIM x).
With such a configuration 3:59 is minimal interval for switching SIM card and 4:01 is maximal interval for switching SIM card. Thanks to this settings SIM cards will be switched approximately every four hours.