Calls from VimpelCom FMC

A rather incomprehensible story happened recently.

Our client, to whom we provide city telephony with the number -1003, complained that they received calls from various operators, and the callers say that they had just missed their number (XXX-1003). The client, respectively, is in shock, for they did not call anywhere, especially in such numbers. CDR records show that calls to these mobiles did not leave our network at all.
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Here the most difficult things began. On the one hand, the client presses and asks to solve the problem. On the other hand, the call is not from our network and not to our network. We decided to catch the numbers to which they allegedly called. Checked which caller ID is determined, it was confirmed that the XXX-1003. Began to write in the support of the big three. Pleased MTS. They got into the problem and tried to find which operator came from the network. But, for some reason, only calls from mobiles to our network were found in the logs (just those when the subscriber of the mobile operator called back to our client and said that he was watching the missed call). Included in the given intervals were not at all. It was suggested that the problem we were considering was either caused by the incorrect operation of a technical device, or fraud or fraud. But the number of our client did not have any paid services, or something like that.

There were already suspicions of mice running around the server, ghosts and the god of telephony.

Suddenly, it turned out that the majority of mobile phone subscribers were waiting for a call from another of our clients who have ANX XXX-0003. It was then that everything began to clear up. We talked with the administrator of the company with the caller ID XXX-0003. It turned out that they use FMC Vimpelcom. We checked the groups of numbers and in one there was a typo, instead of 0003 there was 1003. The problem was solved.

It remains to find out why the operators do not see these calls in the logs. There was a version that subscribers have dual-purpose phones. In this case, the second SIM card of another operator.
From his network, for example, there are callers that MTS, naturally, does not see.
In this case, the client may think that they are calling on his MTS-ry SIM card and calling back from it.
For subscribers requested information about their mobile phones:
- one has something unknown;
- two dvuhsimochnye telephone sets;
- at one odnosimochny.
Thus, this version was deleted from the workers.

There are questions that VimpelCom ignores. Calling a mobile subscriber can be found without knowing the A-number, the main thing is to know its B-number (MSISDN / MSRN) and IMSI / TMSI. B-number was known. Why did not display incoming from FMC Beeline? Why did they respond to requests to Beeline himself that they did not see such calls, although it turns out that outgoing and incoming calls were made through them? There are doubts that they could go to subscribers of other operators, bypassing mobile switches.