Hello. In this year I decided to build my call center and I searched for a reliable sdk tool for my business. I tried out many trial programs, but I found out that .Net sdks are the best ones for my call center business. Ozeki VoIP SIP SDK was finally the chosen one, because to call center applications I could easily add some other features like voice call recording in C# (for the sample program: How to record a voice call? search on the google) Unfortunately the trial versions last only for 20 days, but the support was quite supportive.
Regards,
P.