The Washington Post has first reported on the matter, and has confirmed the claims with sources who have direct knowledge of the endeavor.
The voice interception and recoding program was started in 2009, but it took two years for it to reach full capacity and for a retrieval tool ("RETRO") to be set up to make it fully functional.
Since then, the documents show, five more countries have been targeted with the program. The name of the countries affected have not been revealed to the public, as WP obscured any mention of them in the released documents at the request of US officials.
The recorded phone calls are stored for only a month, and are oldest ones are deleted as newer ones are stored.
"Telephone calls are often thought to be more ephemeral and less suited than text for processing, storage and search. And there are indications that the call-recording program has been hindered by the NSA’s limited capacity to store and transmit bulky voice files," explained Barton Gellman and Ashkan Soltani.
"In the first year of its deployment, a program officer wrote that the project 'has long since reached the point where it was collecting and sending home far more than the bandwidth could handle'."
But as the NSA is expanding its storage capacity, the retention period could change.
MYSTIC is a bulk data collection program. Recorded phone calls that involve US citizens are likely subjected to the agency's "minimization rules."
It also allows agents to retroactively see what an individual has been up to in the 30 days leading to he or she becoming a "person of interest."
"The emblem of the MYSTIC program depicts a cartoon wizard with a telephone-headed staff," the reporters noted. "Among the agency’s bulk collection programs disclosed over the past year, its focus on the spoken word is unique. Most of the programs have involved the bulk collection of metadata — which does not include call content — or text, such as e-mail address books."
Privacy advocates have, naturally criticized the practice, and have expressed their concern that the program will only grow larger.
The NSA has commented the revelation by repeating their usual mantra - "All of the agency's operations are strictly conducted under the rule of law" - and has condemned the reporting of techniques and tools used for "legitimate US foreign intelligence activities."
Reading our newsletter every Monday will keep you up-to-date with security news.
Receive a daily digest of the latest security news.