The communications company Digium, which develops Skype for Asterisk, a software implementation of a PBX, has announced that it will be ceasing this development.
Asterisk is dual-licensed under the GNU General Public Licence and a proprietary licence. Digium describes it as the world's most popular open source telephony project.
Earlier this month, when Microsoft announced its purchase of Skype, chief executive Steve Ballmer told the media that "We will continue to invest in Skype on non-Microsoft client platforms."
In a product notification message, Digium said it had developed Skype for Asterisk in co-operation with Skype.
"It includes proprietary software from Skype that allows Asterisk to join the Skype network as a native client. Skype has decided not to renew the agreement that permits us to package this proprietary software. Therefore Skype for Asterisk sales and activations will cease on July 26, 2011," the message said.
Digium said the change was unlikely to affect existing users. "Representatives of Skype have assured us that they will continue to support and maintain the Skype for Asterisk software for a period of two years thereafter, as specified in the agreement with Digium.
"We expect that users of Skype for Asterisk will be able to continue using their Asterisk systems on the Skype network until at least July 26, 2013. Skype may extend this at their discretion."
Skype is software that is used to make voice and video calls and also chat over the internet. It is a peer-to-peer system.
Microsoft purchased Skype for $US8.5 billion at a time when there were rumours that both Google and Facebook were looking at acquiring the Luxembourg-based company.