Cisco Systems said it's challenging Microsoft's $8.5 billion takeover of Skype at the European Union's top court.

Cisco, based in San Jose, Calif., says it doesn't wish to block the deal that concluded last year but wants European commission regulators to ensure Skype's 700 million customers are not locked into a Microsoft platform. The appeal is on the ground that the deal could prevent video calls becoming as popular as voice calls.

Cisco is the largest provider of equipment for the Internet.

Cisco does not oppose the merger, but believes the European Commission should have placed conditions that would ensure greater standards-based interoperability, Marthin De Beer, the head of Cisco's video conferencing division, wrote in a blog post.

De Beer highlighted Microsoft's plan to integrate Skype with its Lync Enterprise Communications Platform and suggested that this could lock in businesses who want to reach Skype's 700 million account holders to a Microsoft-only platform.

Imagine how difficult it would be if you were limited to calling people who only use the same carrier, or if your phone could only call certain brands and not others. Cisco wants to avoid this future for video communications, he elaborated.

Cisco apparently wants to ensure that Microsoft's voice and video services work with those from rival companies, so that a worker on Skype can hold a call with a worker on Cisco's services.

Making a video-to-video call should be as easy as dialing a phone number, De Beer wrote.

Today, however, you can't make seamless video calls from one platform to another, much to the frustration of consumers and business users alike, he added.

Skype, with roughly 170 million users, is by far the most popular voice-over-Internet service.