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It’s not exactly a case of hacking with the PRISM system. It is an “authorized” form of information retention that has been “legally” in effect for certain systems since Bill Clinton passed the CALEA Act in the 1990s.
Today, telecom providers that offer both traditional voice services and IP telephony as well as ISPs, have backdoors open in their system that allow government intelligence networks like PRISM and the FBI’s DCSnet to monitor certain communications. Originally, CALEA did not include clauses to monitor internet communications but that started to change with introduction of the Patriot Act. This is called lawful (or legal) intercept, which is often abbreviated LI. Both Microsoft and a company called Voip-Pal.com have their hands in this technology, which you can learn about by following the links.