Yes, This is very much amazing and a challenge for mobile networks and standards.
This subject is directly related to NSS, Core and SIM-Card mans. but the effect of it impact the Radio/BSS parts that makes the issue complicated.
We perfom checking with using following lines: (and combine, compare, evaluate all them toghetehr)
1. Using Protocol Analyzer and checking signalling over BSC/MSC in MAP,BSAP,… layer AND checking the cause, response, … of LU request, Roaming request,…
2. Performing Driver Test with some SIMs and having Quality, Scanning, Signalling (Layer2,3,events,…) in attention
3. Checking SIM Cards status with simple readers.
So you see that everything is checked and all of them are compared with recommendations and standards (I was forced to read around 300MB docuemnts and standards)
* again I say that, SIM-cards of Op1 must be rejected by Op2 or Op3 as they don’t have any roaming with eachother in neither same place or other places.
The problem is that:
(1)Why they are rejected in the registration of their own HPLMN!!!! sometimes ???
(2) why in presence of Op1 good power and service, they send request to Op2 and Op3 ???
(3) Why they try again and request again from Op2 and Op3 after some minutes that they had recieved rejection answer from them once !! ???
These three issues deny any standard rule !!!
* May be this subject is a big but hidden problem of some operators in the world. The guys who work in countries which 2 or 3 operators cover same areas without having roaming agreement, may suffer from it or have solution.
Really my knowledge in this issue has come under question 🙂