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Mobile Number Portability (MNP) is a way to enable mobile phone users to change from one mobile network carrier (the donating network) to another one (the recipient network) while retaining their mobile telephone number.

  1. How does this service technically work?
  2. After the portability of a number has been performed, how the routing of the calls is managed?
  3. Is a "ported number" dependent on both donating network and recipient one to work correctly? If the donating network is affected by a fault, does the ported number continue to work?
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  1. Normally, a number from the native range is translated from the telephone number to the SIM's IMSI number. Alien number are recognized by their prefix. With MNP activated, a translation directory is introduced where each entry translates to a carrier entry - usually there is a central directory that's copied/updated by all carriers on a frequent basis. Numbers not in the directory are still recognized by their prefix. Technically, this is somewhat similar to routing table entries in packet networking.
  2. Called numbers are first checked in the translation directory, alien calls are routed to the other carrier, internal calls are translated to their IMSI no. and routed locally.
  3. Not usually - some countries do it this way but most use a central database.
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To expound a little more, an LRN(Local routing Number) is used to identify a carrier that calls for a given TN need to be routed to. If a subscriber changes to another telephone service provider, only an update of the LRN is required to route calls to the new provider.

There are cases in which a donating carrier can be the cause of call faults after a number has ported to a new carrier. If the donating provider does not remove the TN from their equipment or fails to flag the number as "ported", any call generated on that equipment will try to route locally instead of querying the database to determine where the call needs to be routed.

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