This is tricky. We were having this issue as well. We didnt want the end user to hear the ring dial, but then wants them to hear the converstation later on. The only way we though of is getting a knob control of the output skype caller. So while its dialing, you mute it, then when u start the converstation you can put the knob higher up.
Hmm, its still a working progress for us too, but hope this will help you.
Sorry I cant give you more idea, if I come up with more I will try and update you.