We know they listen, this is how.

Big Brother's Big Ear.

German researchers reveal major "flaws" aka eavesdropping tools in cell phone security.

The flaws, to be reported at a hacker conference in Hamburg this month, are the latest evidence of widespread insecurity on SS7, the global network that allows the world's cellular carriers to route calls, texts and other services to each other. Experts say it's increasingly clear that SS7, first designed in the 1980s, is riddled with serious vulnerabilities that undermine the privacy of the world's billions of cellular customers.
The flaws discovered by the German researchers are actually functions built into SS7 for other purposes – such as keeping calls connected as users speed down highways, switching from cell tower to cell tower – that hackers can repurpose for surveillance because of the lax security on the network.

 So it isn't a bug, its a feature.

The researchers did not find evidence that their latest discoveries, which allow for the interception of calls and texts, have been marketed to governments on a widespread basis. But vulnerabilities publicly reported by security researchers often turn out to be tools long used by secretive intelligence services, such as the National Security Agency or Britain's GCHQ, but not revealed to the public.
"Many of the big intelligence agencies probably have teams that do nothing but SS7 research and exploitation," said Christopher Soghoian, principal technologist for the ACLU and an expert on surveillance technology. "They've likely sat on these things and quietly exploited them."

I'd go further and speculate that the big intelligence agencies ARRANGED this as far back as the 1970's, forward to today.

Just my tinfoil hat speculation.

The Phantom