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Re: Setting up Messenger using a duplicate SIM
If its their offer why didn't they set it up for you and explain its use?
Mobile network operators run a subscriber database, the HLR .
Every SIM has a unique identity, the IMSI . The HLR associates an IMSI with a subscriber by storing IMSI and subscriber data together.
When switching on a mobile phone, and afterwards in regular intervals, the phone registers its SIM with the network (a "location update" in telco speech). During the register procedure, the mobile network performs authentication and authorization against the HLR and marks the IMSI as being active. The authorization will fail if an IMSI is already marked as active. If the authorization failed, the network won't allow the mobile to access its services.
This has been built on purpose into the network architecture:
since the network can deliver a call to one device only. There is no default functionality to have several devices ringing for calling a single number. (This is possible, but requires additional systems.)
to prevent hostile take-over of numbers (delivering a call to a party it is not intended for).
to prevent fraud, e.g. by not being a paying subscriber at all or, in your case, by using two devices on one contract.
Some network operators offer special products to overcome this on-purpose limitation. For example you may get an additional SIM, coded to your telephone number, which disables your first SIM when switched on. This is useful if you have a phone installed in your car requiring a separate SIM. In this case, there is logic in the HLR which supports switching SIMs.