O2 UK before 2025-05-19 allows subscribers to determine the Cell ID of other subscribers by initiating an IMS (IP Multimedia Subsystem) call and then reading the utran-cell-id-3gpp field of a Cellular-Network-Info SIP header, aka an ECI (E-UTRAN Cell Identity) leak. The Cell ID might be usable to identify a cell location via crowdsourced data, and might correspond to a small physical area (e.g., if the called party is in a city centre). Removal of the Cellular-Network-Info header is mentioned in section 4.4.19 of ETSI TS 124 229.

O2 UK through 2025-05-17 allows subscribers to determine the Cell ID of other subscribers by initiating an IMS (IP Multimedia Subsystem) call and then reading the utran-cell-id-3gpp field of a Cellular-Network-Info SIP header, aka an ECI (E-UTRAN Cell Identity) leak. The Cell ID might be usable to identify a cell location via crowdsourced data, and might correspond to a small physical area (e.g., if the called party is in a city centre). Removal of the Cellular-Network-Info header is mentioned in section 4.4.19 of ETSI TS 124 229.