Hi! In this year I tested some sdks and ready-to-use softphones because I needed a reliable softphone which is cost-effective. I decided to select from sdks and not from ready-to-use softphones because they are more flexible and provide high compatibility if you need further development. I wanted to wait the end of testing because I wanted to be sure about choosing the proper sdk. Finally I have chosen Ozeki Voip SIP SDK because of the good support and high voice quality.

Sample program:
http://www.voip-sip-sdk.com/p_354-im...swer-voip.html

Unfortunately it seems to be expensive but due to professional license you can also develop an autodialer system and IVR and thus create a whole call center with good sound quality.