What is "international roaming" and how does it use your phone number?

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International roaming is a service that allows mobile phone users to continue using their existing mobile phone number and services (voice calls, SMS, and mobile data) when they travel outside the geographical coverage area of their home mobile network and connect to a network in another country. Essentially, it extends the reach of your home mobile service across international borders.

How it Works and How Your Phone Number is Used:
International roaming is a complex technical and commercial arrangement between your "home mobile operator" (the company you have your primary mobile contract with, e.g., Grameenphone in Bangladesh) and "visited mobile operators" in other countries. Your phone number is central to this entire process:

Home Location Register (HLR) - Your Permanent Record:

Every mobile subscriber's permanent information is stored in a database called the Home Location Register (HLR) within their home network. This database contains crucial details like your phone number (Mobile Subscriber ISDN Number - MSISDN), your International Mobile Subscriber Identity (IMSI – a unique identifier for your SIM card), the services you are subscribed to (e.g., voice, SMS, data, roaming enabled), and your authentication keys. Your phone number is the primary key to access this permanent record.
Visitor Location Register (VLR) - Your Temporary Local Record:

When you travel to a foreign country and switch on your phone, your romania phone number list device searches for available local mobile networks.
If your home operator has a roaming agreement (a wholesale commercial and technical arrangement) with one of the visited networks in that country, your phone will attempt to connect to it.
Once your phone connects to a visited network, that network creates a temporary record for your device in its Visitor Location Register (VLR).
Authentication and Authorization using Your Phone Number:

The VLR in the visited network communicates with your home operator's HLR (using your IMSI, which is linked to your phone number) to authenticate your identity and verify that you are authorized to use roaming services. The HLR checks your subscription status and confirms that you are allowed to roam.
Once authenticated, your home HLR updates its record to reflect that your phone is currently registered with a specific VLR in the visited network. This temporary record in the VLR and the updated HLR entry ensure that all incoming calls and messages to your original phone number can be correctly routed to your current location.
Seamless Service Delivery:

After successful authentication, the visited network provides you with services (making/receiving calls, sending/receiving SMS, using mobile data) as if you were on your home network, but using their infrastructure. Crucially, you continue to use your original phone number.
When you make a call while roaming, the visited network routes the call. If you call home, it sends the call back to your home country's network via international transit services. If someone calls you, the call is first routed to your home operator, which then queries its HLR to find your current VLR location and forwards the call to the visited network.
For SMS, similar routing principles apply. When you send an SMS, the visited network passes it to your home network, which then delivers it to the recipient. When you receive an SMS, your home network contacts your current visited network and routes the message accordingly.
Billing:

The visited network records all your usage (calls, SMS, data) in "Transferred Account Procedure" (TAP) files and sends these records to your home operator.
Your home operator then calculates the charges based on its roaming rates (which are usually higher than domestic rates due to the wholesale agreements with foreign operators) and adds them to your regular bill. You pay your home operator, not the foreign network directly.
In Bangladesh, all major mobile operators like Grameenphone, Robi, Banglalink, and Teletalk offer international roaming services to their subscribers, facilitated by these intricate global roaming agreements. Your phone number is the consistent identifier that makes this entire cross-network functionality possible, allowing you to remain connected with your friends, family, and work using your familiar number, no matter where you travel.