Need clarification about the VOIP

Question

this is my continuation of my previous question Java SipServlet to build VOIP phone calls (between Computer and analog phone/mobile)

I've stumbled across a opensource softphone to use it to try make a phone call but the software requires the SIP provider. And according to my searching on google so far, purpose of SIP phone is to make a phone call between two SIP caller and callee which mean those two people have to have registered with one of a SIP provider. But my current goal is to make VOIP to call to my ordinary mobile phone or land line phones (no matter what phone providers they are using), and software like Skype or Google phone can already do that. I'm right now researching about sip servlet, am I going into a wrong direction right now?

Answer 1

For a SIP call to go to a cell or land line phone you need a piece of hardware called a VoIP Gateway that translates between the SIP protocol and a digital or analog phone line. I am sure Skype and Google have an infrastructure setup that hooks directly to the big carriers to handle this.

By the way, Skype is not SIP but uses their own proprietary VoIP protocol.

What is Google phone? Are you referring to Google Voice?