Need to intergate my software with VoIP, advice needed

Hi guys. I decided to move my office phones to VoIP, finally.
I'm testing Callcentric so far... to early to say how good it is yet.
Here's my question. I have my own office software, written in c#. I need to send a following information to this program every time my phone rings - incoming #, and which one of my lines was used for that. Also, I'd like to be able to have "Call" button from my client database.
Now I understand that it's not a developer's forum, but maybe you guys know the answer. Should I stick with softphones? The ones that offer their own API? If yes, which ones? Or maybe regular IP phones, maybe from a higher end, offer some kind of way to handle this kind of requests?

Any advice is much appreciated.
Thanks