When a subscriber travels outside his or her home operator service area, we say a subscriber is roaming onto another network. It is important SMS messages are still delivered correctly when a subscriber is roaming, and as such the foreign network’s VLR communicates with the HLR of the home network to ensure the location of the subscriber is maintained at all times. It should be noted that even while roaming a subscriber will always send a message via their home SMSC. In the GSM world it is very rare for a subscriber to be charged to receive a message while roaming.
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