Officials at the FBI and the Cybersecurity and Infrastructure Security Agency are warning Americans to stop using text messaging and calling services that don’t provide end-to-end encryption after an unprecedented cybersecurity attack. According to NBC News, the warning was issued Tuesday during a call with reporters that included an unnamed senior FBI official and Jeff Greene, the deputy director for cybersecurity at CISA.

During the call, officials confirmed that Chinese hackers had attacked Verizon, AT&T and Lumen Technologies to spy on those companies’ customers. As a result of the attack, the hackers may now be able to access communications made via unencrypted text messages or phone calls.

The warning specifically affects anyone who sends text messages between an iPhone and an Android device. While messages between iPhones – which show up as a blue iMessage on those phones – and Google messages sent between two Android phones are automatically encrypted, this isn’t the case if each user has a different phone type.

Messages sent between an iPhone and an Android device are vulnerable. Officials therefore suggest that those who need to send a message this way should use an encrypted service such as WhatsApp or Signal. While the data associated with encrypted messages can still be stolen by hackers, Greene says encryption makes it impossible for the hacker to see what is contained in the data.

So far, officials have not said how many Americans are at risk from the attack, and Greene said there is no timeline for “when we will completely drive out the hackers.”