When your SIM attaches to a network, our HLR pushes the list of allowed APNs (during ISD) to the visited network's SGSN, and some SGSNs are intelligent enough to use this information in case an incorrect or no APN is configured in the device.
This feature is solely dependent on the visited network infrastructure and currently only a few operators are confirmed to have this feature in their SGSN - so this functionality is not guaranteed to work. Therefore, it is still always essential that the correct APN must be configured in the device for the data session to work.
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Shahzad Ismail
Head of Knowledge Management and Community Engagement, Sinch