Millions of cellphone users across the U.S. got an alert Wednesday from the Federal Emergency Management Agency and FCC to ensure emergency alerts reach the public at a national level. Phones received both a test message and made a loud alert noise.

However, the testing system made prisoners hiding phones an easy target for prison guards.

A New York State Prison official tells us they confiscated two phones at Sing Sing Correctional Facility during the test. Additionally, a source at FCI Coleman Low in Florida says they too confiscated two phones during the emergency test.

It's unclear how many other prisons faced similar circumstances -- we called around in Arizona, California, Illinois and others -- but you gotta imagine it was a problem nationwide.

A source in Nevada tells us most of their prisoners were already aware the alert was gonna happen, likely turning their phones off, so they didn't confiscate any devices.

When reached for comment, the BOP told us, "The Federal Bureau of Prisons does not elaborate on specific internal security procedures for safety and security reasons."

Unsurprisingly, we're told phones behind bars have become a growing problem, and officials have implemented new tactics in trying to locate them -- including phone-sniffing dogs called "E-Dogs."