Swedish member of Kaspersky Lab s global research and analysis team (GReAT) presented a presentation called Exploiting Telco Support Teams for Fun and Profit He explained how Swedish telcos ask only for a bare minimum of information from callers before agreeing to make account changes to specific numbers. This has led to real-world attacks where victims have found their mobile phone calls hijacked and redirected to a rogue number. The ease of which someone can redirect the calls is a dream for a stalker, or for someone interested in sniffing out personal information on an individual’s contacts and calling habits.
Source: https://threatpost.com/social-engineering-telcos-phone-hijacking/144495/