Okay, so what is in your log file around the time of the mystery change to UNDEV (which looks like a corrupted UNDEF)
Alright, that’s a string type Item.
Is calledNumber a string? I don’t know, it was derived from a list object. I think I’d make sure -
Flag_LetzteAusgendeNr.postUpdate(calledNumber.toString)
Edit - is there some confusion with Item Flag_LetzteEingehendeNr ?
That one is defined with invalid syntax
string / String
Further Edit -
I’d be pretty sure that your rule is running perhaps when you don’t expect it. And then you may run into issues examined here -
where you might very well get UNDEF for caller number when there is no active call etc.
That’s assuming the state really is UNDEF. Your logging should reveal this.