First let us separate the two voice message:
Mobile Subscriber is switched OFF – the message is active when the VLR successfully received the IMSI Detached message before the Mobile power down. Or this message can also be activated if the mobile failed to perform Location Update (LU) on a specified period of time. this scenario is also know as “Implicit IMSI Detach”.
Mobile Subscriber is Out of Coverage Area – The message is activated if the MS does not perform IMSI detach but cannot be paged by the network. I have personal experiences of these cases whenever me and my friends go for outdoor adventures, like trekking, camping..etc. :). Wherein we are being served by Multi path fluctuating signals.
So, to hear both voice messages in consecutive call attempts is really something to investigate.
I think MSS is the same as MSC – Mobile Switching Center.
By the way Bob, it is very intriguing to raise the Mapping issue between 2 operators.
But the way your explanation goes, it seems that the mapping issue is internal to any of the operator, between their BSC and MSC code Mapping alone. The voice messages are pre-determined independently from each operator.