- Some discussion on SIP and C#
- Windows call manager and outlook dialer
- Also, as far as Asterisk goes, there were some links on this page.
I am sure that some other hits would be found on at least one of the following links:
I hope at least some of that helps. As far as COM Interop goes, there is an interesting article at Code Project, or at MSDN. For C Library interop with .NET, see this page.