Hi! I began to use Ozeki voip sip sdk to create a call center for our business projects. The sound quality was extremely important for my boss and we got it with this sdk. We continued to develop it by adding call recording in C#, the sample program for it:
http://www.voip-sip-sdk.com/p_362-voip-call-recording-voip.html was quite instructive and useful. The trial version lasts only 20 days but is really good, and the support is also cooperative.