There are several. slfphone Ekiga gstreamer
I use Ekiga a lot. We had written some for customising lots of things in Ekiga. Rather than skype, Imo Ekiga would be a better bet.
Ekiga is great. I may also suggest Qutecom, formerly known as wengophone ( http://www.qutecom.org/), a easy to use and great looking softphone.
However, the objective of our appliance project is not to provide a SIP softphone for a desktop. Objective is to have a appliance to which, one could connect their regular phone instrument using a voip adapter such as Linksys PAP2T (http://www.cisco.com/en/US/products/ps10029/), make and receive calls from anywhere in the world. Think of it as a affordable alternative to Vonage like VoIP solutions which are very expensive and have vendor lock ins.
Typical use case would be: Folks who have their son/daughter's/relatives abroad, need high volume calling, and do not want to start their desktop and use some app to make and receive calls. They could just pick up their landline like instrument, dial a number which lands either on one's skype application on computer/cellphone/landline. Skype provides skypein number and skype out credits which makes it capable of receiving and making calls to landlines/cell phones, and provides very affordable calling plans than subscribing to a SIP provider's plans. Similarly, google has made calling US numbers very affordable with their google voice/gtalk integration after acquiring grand central project. Thats the reason for choosing skype. If there is a alternate libre solution aailable, I would be more than glad to use it as it would save lot of complexities integrating skype.
Thanks gs
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