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

[suhashini25]

"International roaming" is a service that allows a mobile phone user to continue to use their existing mobile phone number and services (voice calls, text messages, and mobile data) when traveling outside the geographical coverage area of their Home Public Mobile Network (HPMN). This is made possible through commercial agreements between the user's home network operator and foreign mobile network operators, known as Visited Public Mobile Networks (VPMNs).

Essentially, it's like your home mobile service "lending" you to a network in another country so you can stay connected.

How it Works and How it Uses Your Phone Number:
Your Phone Number Remains Constant:
The core benefit and defining characteristic of international roaming is that you retain your familiar phone number (the MSISDN - Mobile Station International Subscriber Directory Number). People can still call or text your regular number, regardless of which country you are physically in. The network handles the routing to your current location.

The Role of Your SIM Card and IMSI:

SIM Card (Subscriber Identity Module): Your SIM card is the key. It contains your International Mobile Subscriber Identity (IMSI). The IMSI is a unique identifier (typically 15 digits) that identifies you as a subscriber of your home network. It comprises:
Mobile Country Code (MCC): Identifies the country of your home network.
Mobile Network Code (MNC): Identifies your specific home mobile operator within that country.
Mobile Subscription Identification Number (MSIN): Uniquely romania phone number list identifies you within your home operator's network.
Authentication and Location Update: When you arrive in a foreign country and turn on your phone (or disable airplane mode), your phone searches for available local mobile networks (VPMNs).
It then transmits your SIM's IMSI to the visited network.
The visited network recognizes that the IMSI belongs to a foreign subscriber and queries your home network's Home Location Register (HLR).
The HLR is a central database in your home network that stores all the essential information about your subscription, including your current location (even if you're roaming), services you're subscribed to, and whether you're authorized to roam.
If there's a roaming agreement in place between your HPMN and the VPMN, and your account is provisioned for roaming, your home network authenticates you and informs the visited network that you're an authorized roamer.
The visited network then creates a temporary record for you in its Visitor Location Register (VLR), allowing you to access its services. Your HLR is updated to reflect that you are now registered with that specific VPMN.
Call and Data Routing:

Incoming Calls: When someone calls your home number, your HPMN consults its HLR, sees that you are registered with a specific VPMN, and then routes the call to that visited network. The visited network then delivers the call to your phone.
Outgoing Calls/SMS/Data: When you make a call, send an SMS, or use data while roaming, your phone communicates with the VPMN. The VPMN then routes the traffic back to your HPMN (often via international transit carriers and clearing houses) for billing and further routing to the final destination.
Benefits and Downsides:
Benefits:

Seamless Connectivity: You stay connected with your home number, making it easy for friends, family, and colleagues to reach you without needing a new number.
Convenience: No need to switch SIM cards or manage multiple numbers. Your phone behaves largely as it would at home.
Familiar Services: You can often access your voicemail, banking alerts, and other services tied to your home number.
Downsides:

High Costs: The primary drawback is often the significantly higher cost for voice calls, SMS, and especially mobile data, compared to local rates or even international direct dialing from your home country. These costs arise from the wholesale agreements between operators.
Network Performance: Data speeds and call quality can sometimes be inconsistent, depending on the quality of the visited network and the roaming agreement.
Bill Shock: Without careful management (e.g., using roaming packages or turning off data roaming), consumers can face unexpectedly high bills upon returning home.
In Bangladesh, mobile operators like Grameenphone, Robi, Banglalink, and Teletalk offer international roaming services. Users typically need to activate roaming before travel and often choose specific roaming packages to manage costs. While convenient, alternatives like local SIM cards or eSIMs are often considered more cost-effective for extended stays abroad.