Information Disclosure on Telecom Service Level Degradation, SMS Usage Affected with No Prior Notice, Late January 2024

Background

As a China Unicom subscriber, Li Ao keeps mobile service in contract with China Unicom Xuzhou Subsidiary, a subsidiary of China Unicom Jiangsu Corporation. In late January 2024, he suddenly noticed that no SMS messages can be received, with no prior notice to any possible service suspensions or disruptions by SMS, and email which is registered as a contact method. The subscriber complained to the company by getting in contact with their designated customer service channel on Web, with receiving helpless messages in the context of telling the current subscriber status and the instructions asking to change the mobile device.

The subscriber later complained to the service acceptance area serving as the most privileged service center in Xuzhou. Two representatives accepted the complain, and later determined the possible cause. Upon confirmation of the technician, a representative confirmed with the subscriber that a service in name of incoming call barring affected the usage of SMS, and therefore he cannot interfere with the internal system to solve the service issue. He refused to forward the subscriber's request asking for further confirmation on the nature of setting up such restriction.

Initial Appearance of the Issue

The issue initially happened with the following appearance:

-- No incoming calls being rang on user's GSM modem, but China Unicom announces that the phone subscriber can't be reached at the moment, for calls placed on that number. The original English transcript is "Sorry, the subscriber you dialed cannot be connected for the moment, please dial again later." Before this issue happens, calls placed on the subscriber's number will be reported as the subscriber does not have voice calling functionalities, transcribed as "Sorry, the subscriber you dialed doesn't have voice service, please try other numbers."

-- SMS messages sent to the subscriber's number no longer be able to be received. Message delivery status does shows as failed.

-- Calling functionalities appeared to be OK. Outgoing calls can be placed and can be connected, same to other subscribers with normal setup.

-- Outgoing SMS messages worked as usual, and can be delivered to the recipients and delivery status can be reported back to subscriber's modem.

-- No prior notifications by SMS was sent, to the subscriber's phone number. Despite there is a wrong number registered on the subscriber's identity profile as contact number, this information field is believed which is rarely used as a contact method of the customer.

-- No prior notifications by email was sent, to the email address registered on the subscriber's identity profile as contact email address. Interestingly, contrary to contact phone number registered, with every service change including replacement SIM card issuance; turning on/off calling, messaging, and other services; and purchasing data packs and calling minutes being imposed into the BSS system (cBSS in its name), the system will automatically send a copy of the electronic statement announcing service changes to customer's phone service, to such the registered email address.

-- The cBSS Business Supplement System (translated from original name in Chinese "cBSS yewu zhicheng xitong") showed up that the subscriber's phone service is running normally, and no indications of any abnormal service behaviours was reported by the system.

There is a service appears to be used to flush the cached subscriber data in the cellular network or the HLR containing the respective subscriber information or both, called clean card and reuse, existed in the cBSS system and can be imposed on request of the customer. Li Ao asked to perform such operation, and the issue still persisted after being reported the flush is successful.