I am the owner of a business which I started only a few months ago. I would also like to add another availability like a webphone. I am a person who does not know much about computer technology and that is why I ask for help. I found a site that says with an SDK one can build an own webphone. It is called Ozeki VoIP SIP SDK (http://www.voip-sip-sdk.com/)
Could you please tell me about this SDK, did you hear about it, use it etc.?
This is a very obscure topic and something which requires quite a lot of specialist knowledge. I'm afraid to say that, although I may be wrong, I doubt you will find anyone here who has any experience with this SDK.
I've never heard of this SDK, but then again, I've never heard of any of the competitor's SDKs either. I would suggest that you download the SDK and try it out to see if it suits your needs:
