A-number and B-number on IoT SIM cards: what do they mean?

Anyone who opens the usage overview or a CDR (Call Detail Record) of an IoT SIM card will often find two fields for SMS and VOICE: the A-number and the B-number. The A-number refers to the sender or caller and the B-number to the destination. Which fields are visible differs per provider and platform. These fields show which numbers a device communicates with and help you investigate invoice costs and detect possible misuse or faults.

What is an A-number?

The A-number is the number of the party that sets up a connection. For an SMS, this is the sender, and for a voice call, the caller.

The A-number on an IoT device

When a device sends a message itself, the A-number is the MSISDN of the SIM card in that device: the phone number the network provider links to the SIM card. A tracker that sends an SMS when a trailer is moved without permission, or a lift emergency phone that calls the control room, appears in the traffic record with the MSISDN of its own SIM card as the A-number.

Which other names will you come across?

Providers and platforms use different terms for the same field. In CDRs and portals, the A-number is also listed as calling party, calling number or originating number.

What is a B-number?

The B-number is the number of the receiving party: the recipient of an SMS or the number called. In portals, this field is also called called party, called number or terminating number.

Which B-numbers will you come across in IoT?

For outgoing IoT traffic, the B-number is the number to which a device sends its messages or makes its calls. Think of the mobile number of a technician who receives a fault notification, the number of an SMS gateway that forwards messages to an application, or a supported short code of an SMS service. For a lift emergency phone, the B-number is the number of the control room.

When are the roles reversed?

For incoming traffic, the device is the B-party. When a platform sends a wake-up SMS or configuration command to a device, the platform’s sender is the A-party and the MSISDN of the SIM card is the B-number. That sender can be a gateway number, but also a short code or sender name. Providers distinguish between MO (mobile originated) for traffic from the device and MT (mobile terminated) for traffic to the device.

Do A- and B-numbers also apply to data traffic?

A- and B-numbers describe the sender and destination for SMS and VOICE. For data traffic, a device sets up a data session via the APN and communicates with servers via IP. The traffic record usually contains the APN, data usage and session duration. An MSISDN can identify the connection in that record, but there is no telephone destination number. The server destination is not always visible. For a more complete picture, compare SMS and VOICE records with the data session details.

Where do you come across A- and B-numbers in practice?

CDRs and usage overviews

SMS and VOICE records usually contain the sender, destination, time, direction and, for VOICE, the call duration. Which fields are available differs per provider. This lets you investigate the traffic per SIM card; an SMS record does not automatically confirm that the message was delivered.

Billing

The B-number can partly determine the rate. Depending on the subscription, national numbers, international numbers, short codes and premium services can fall into different rate groups. To explain an unexpected invoice item, also review the subscription, roaming, traffic direction, call duration and the number of billable SMS segments.

Fraud detection

A device that unexpectedly sends messages to international or premium numbers may be working differently than intended. This can point to a configuration error in the firmware or to a SIM card being used elsewhere. With thousands of SIM cards, monitoring destination numbers and usage helps to identify such anomalies in time.

Troubleshooting

If the platform no longer receives notifications from a device, the records can show whether the device is still sending SMS messages and to which B-number. Traffic to an old gateway number can indicate a configuration error. If the configuration can be adjusted remotely, a technician visit may not be necessary.

What should you consider if your devices use SMS or voice?

For devices that depend on SMS or VOICE for alarms, wake-up or remote configuration, a few choices made before rollout determine whether this works reliably in the field.

Does the SIM card have an MSISDN?

For regular SMS via mobile phone numbers, the SIM card uses an MSISDN as sender or destination. When ordering, check whether the SIM cards have the required numbering and SMS functionality. An MSISDN alone does not guarantee reachability: some IoT subscriptions only support SMS via the provider’s own platform.

Are SMS and VOICE active on the subscription?

The required SMS and VOICE services must be supported and activated. Check this per SIM card profile and traffic direction. For VOICE, the device and the networks used must also be suitable, for example for VoLTE.

Does SMS work in the rollout countries?

Devices deployed internationally often use networks of other operators. Whether SMS works depends on roaming agreements, local restrictions and support by the device and network. Before rollout, test in each target country whether SMS works via the intended networks and routes in both required directions.

Do you want to restrict traffic to fixed B-numbers?

Where the provider supports this, you can restrict outgoing SMS and voice to fixed B-numbers, such as your own SMS gateway and control room. This reduces the risk of unwanted destinations and costs. Data traffic and incoming traffic are not automatically covered by this restriction and require separate settings.

How does Thingsdata set up SMS and VOICE on IoT SIM cards?

A- and B-numbers help to analyze the SMS and VOICE traffic of devices, explain costs and identify unwanted destinations. For data traffic, the APN and session details provide insight. Thingsdata helps set up both. With multiple connectivity partners and access to more than 650 networks, Thingsdata selects a solution that fits the locations and application. The required services and, where supported, permitted B-numbers are configured per project. Thingsdata Control provides insight into the available usage per SIM card; how current and detailed this is depends on the connected provider. With suitable eUICC hardware and SGP.32, operator profiles can be managed and switched remotely, without a physical SIM card swap. Thingsdata offers connectivity in more than 180 countries and is ISO 9001 and ISO 27001 certified.

Would you like to know which SMS and voice settings suit your devices, or would you like fixed B-numbers configured? Contact us at +31-85-0443500 or info@thingsdata.com.