Numbers transferred or “ported” from EE to any new networks, including Three, O2 and Vodafone, were affected, the Vodafone spokesperson added, as three of the digits in the mobile number still identify them as numbers “EE”. This means that even though they are no longer EE customers, calls are briefly routed through EE’s infrastructure.
Source link: https://www.bbc.co.uk/news/technology-66708574