VOIP ActiveX/COM Object

I am looking to integrate VOIP into a windows application I am writing, does anyone have experience in writing a VOIP application which would allow for simple, dialing, and hang-up of a phone number through software?

Can this be done generically so that you could have the freedom to work with any provider? Or, do you have to pick a provider upfront and write the app. to support only that provider?

Thanks.