This 2011 presentation by the head of GCHQ’s Network Analysis Centre outlines the agency’s interest in exploiting telecommunications companies, namely to “get at the data before it is encrypted”: see the Intercept article Operation Socialist: The Inside Story of How British Spies Hacked Belgium’s Largest Telco, 13 December 2014.
This GCHQ document from June 2010 describes cooperation between members of the Five Eyes alliance, particularly Canada’s CSEC, on the technical analysis of international mobile phone networks: see the Intercept article Operation Auroragold: How the NSA Hacks Cellphone Networks Worldwide, 4 December 2014.
This 26-page NSA document, originally presented at 2012’s Five Eyes SIGDEV conference, gives a detailed overview of the agency’s information gathering on world mobile phone networks. Some intercepted technical and contact details have been redacted: see the Intercept article
Operation Auroragold: How the NSA Hacks Cellphone Networks Worldwide, 4 December 2014.