Let me give here another example:
These cases are my real experiences when I worked with another operator. I hope they could draw a picture
Prepaid SUB A in MSC-S is called, MSC-A dose HLR enquiry, after all it receives PRI (provided MSRN) with CFN (Call forward Num) in Out of reach or OFF situation. Now paging receives no answer–>now call forwarding is processing but before it, IN should be trigged to check balance. Unfortunately she doesn’t have money as usual 🙂 Here IN has not been correctly configured to create a cause with proper announcement. So sends something to mss indicating “hey MSS stop this procedure, her balance is 0$”! Now MSC knows, should return a reason to originating MSC by ISUP message- now MSC is wondering which causes code should be used? In fact MSC should do a mapping between CAP and ISUP. The best solution is using this cause code for ending this call –> Release due to other failure. Hey IN this is not a real reason, then I asked IN Guys to consider this Situation and they did it. Now if somebody calls her, correct Announcement with proper cause code will be sent and played.
Another Example–>Sometimes you call your fiend Homed in another operator –> paging is unsuccessful due to some reason in BSSAP. Now it is time MSS does a Mapping between a BSSAP’s causes to ISUP message and sends it back to originating MSC (your PLMN ) BUT there is no correct mapping between BSSAP and ISUP or ISUP can’t support such cause code so you receives RELEASE – Normal Unspecified. Oh my god what is the reason??
This also happens between MAP and ISUP cause code, usually interconnection between two operators use some TMSC (Transit MSC) like Gateway. We use 6 in our biggest city, when I keep calling a subscriber who is out of reach or off some nice scenarios happen. You can do an interesting job to validate another operator, forward another operator MS to a wrong number and keep calling to it , let see what happens especially if you use more than 1 TMSS or GMSS
Sorry if I couldn’t explain well due to my poor English and knowledge