[Asterisk-Dev] Tie VOIP into my web application

[K J]

I want to tie my web application into a VOIP service so that users can
call each other.  I want them to interface with my application's
username system.

On the receiving user's end, he can either receive the call using a
VOIP phone, or windows application (like skype).

I would use Skype's API, but it appears I need to use their username
system, and not my own.

My question is, what software/hardware solutions would I need to do
this?  Any suggestions/feedback would be greatly appreciated.