Magnus,
If you go to the mailbox/Primary Message Notification/1/timers and limits/ Change the value of "Personal Number No Answer" to 5. This should accomplish what you want. You may also want to consider lowering "Number Called Busy" to 1-2 mins and "number of call attempts" to 2. This will allow it so that if someone is on the phone they will get another call in 1-2 mins giving them the chance of answering it before rolling to the next cascade.
-Travis