US and British intelligence agencies reportedly hacked into a major manufacturer of SIM cards to gain access to large amounts of data worldwide.
US website Intercept said the information came from former US intelligence contractor Edward Snowden.
The company, which is alleged to have been hacked is the Netherlands-based Gemalto, which produces more than two billion "subscriber identity modules - or SIM cards - a year.
Intercept claims that the hack organised by Britain's GCHQ and the US' National Security Agency took place in 2010.
Neither agency has commented on the allegations.
Franco-Dutch company Gemalto did not say if it was hacked.
"From what we gathered at this moment, the target was not Gemalto, per se - it was an attempt to try and cast the widest net possible to reach as many mobile phones as possible," a Gemalto spokeswoman said in an email.
"We take this publication very seriously and will devote all resources necessary to fully investigate and understand the scope of such highly sophisticated technique to try to obtain SIM card data," she said.
Gemalto is believed to be one of the world's largest manufacturers of SIM cards and produces service providers with encryption codes to keep the data on each phone private.
Intercept said GCHQ targeted the company's engineers to gain information that gave them access to the company's networks.
Encryption keys were then reportedly stolen that allow the data that passes between mobile phones and cell towers and unscramble calls, texts or emails to be decoded.
The hacks are said to have taken place in 2010 and 2011 and led to the interception of 300,000 keys for mobile phone users in Somalia, as well as wireless network providers in Iran, Afghanistan, Yemen, India, Serbia, Iceland and Tajikistan.
A GCHQ spokeswoman said they do not comment on intelligence matters.
But she said: "Furthermore, all of GCHQ's work is carried out in accordance with a strict legal and policy framework, which ensures that our activities are authorised, necessary and proportionate, and that there is rigorous oversight, including from the Secretary of State, the Interception and Intelligence Services Commissioners and the parliamentary Intelligence and Security Committee.
"All our operational processes rigorously support this position. In addition, the UK's interception regime is entirely compatible with the European Convention on Human Rights."
