A cyberattack over the weekend has disrupted the network of British telecommunications giant Lyca and prevented customers from buying more minutes.

Lyca calls itself the world's largest international mobile virtual network operator with over 16 million customers. They offer pay-as-you-go SIM cards across 23 countries in Europe, Africa and Asia.

This week, the company said it began investigations after it became aware of issues customers were having buying more call minutes and making international as well as national calls.

“The issues affected all Lyca Mobile markets apart from the United States, Australia, Ukraine and Tunisia,” the company said.

“Our number one priority is ensuring the safety and security of our customers’ data, and we are urgently investigating whether any personal information may have been compromised as part of this attack. We are confident that all our records are fully encrypted, and we will keep customers updated on the outcome of our investigation as we work with our expert partners to establish the facts.”

The company hired technical experts to help with the response alongside law enforcement agencies.

Lyca said the mobile telecommunication services affected by the attack have now been restored in all of their markets but that “there are some operational services that are yet to be fully resolved.”

The company did not respond to requests for comment about whether it was a ransomware attack, and no attackers have come forward to take credit for the incident.

Telecommunications firms are under frequent attack due to the troves of sensitive personal information they hold on customers. The companies are targeted both by criminal hackers and nation-states.

Telecoms across the Middle East, Southeast Asia and Africa have faced nation-state attacks in recent months alongside more targeted attacks by cybercriminals on French giant La Poste, Australia’s Optus, U.S.-based Charter Communications and Lumen Technologies. Several state-owned telecoms have also dealt with cyberattacks this year, including ones in Guam and Tonga.

On Monday, the European Telecommunications Standards Institute said hackers stole a database identifying its users.