British mobile virtual network operator Lyca Mobile has confirmed it suffered a cyberattack that disrupted its operations.

Lyca Mobile is the world’s largest virtual network operator with over 16 million customers offering low-cost, data and voice services across 23 countries around the world.

Lyca Mobile customers started experiencing problems last Friday, with some users reporting that they’d been unable to make mobile calls or send SMS messages, while others were unable to get through customer support or to top up their credit via Lyca’s website.

At the time, the operator blamed the situation on technical difficulties but now it has confirmed that it was hit with a cyberattack that “impacted some national and international calling,” as well as customers’ and retailers’ ability to access top-ups through Lyca Mobile’s channels. The issue affected customers of all Lyca Mobile markets except for the United States, Australia, Ukraine and Tunisia.

The company didn’t reveal the nature of the security incident but said it has launched an investigation and is working to determine whether any data was stolen.