Cisco has said it plans to appeal against the European Commission's approval of Microsoft's £5.3bn acquisition of Skype, due to concerns around video-calling interoperability.
Microsoft announced it planned to acquire Skype last May, and the deal was approved by the commission in October.
However, networking company Cisco has expressed concern in a blog that Microsoft's plans to integrate Skype exclusively with its Lync Enterprise Communications platform will create vendor lock-in.
"For the sake of customers, the industry recognises the need for ubiquitous unified communications interoperability, particularly between Microsoft/Skype and Cisco products, as well as products from other unified communications innovators," wrote Marthin De Beer, senior vice president of emerging business group, Cisco, in the blog.
"Microsoft's plans to integrate Skype exclusively with its Lync Enterprise Communications Platform could lock in businesses that want to reach Skype's 700 million account holders to a Microsoft-only platform."
De Beer argues that for video calls to become "the next voice" the communications industry needs to adopt a model of interoperability around open standards.
Messagenet, a European VoIP service provider, has joined Cisco in appealing the merger to the General Court of the European Union.
"The appeal is about one thing only: securing standards-based interoperability in the video-calling space," wrote De Beer.
"Making a video-to-video call should be as easy as dialling a phone number. Today, however, you can't make seamless video calls from one platform to another, much to the frustration of consumers and business users alike."
Skype was founded in 2003 and purchased by eBay for $2.6bn (£1.6bn) in 2005. It was then acquired by an investment group in 2009, led by Silver Lake.