Session Initiated Protocol is the open standard for Voice Over Internet Protocol. With SIP, you are able to use open standard devices such as analog telephone adapters, or Wifi phones. The user will typically have to provide a UID, Password, and SIP server for the item to work. Once properly setup, the device can make and receive telephone calls at a reasonable rate, or maybe in some cases – free.
Many SIP providers will also offer a SIP address which is sip:ID-Number@carrier. Yes, this looks like an email address, but the difference is the initial SIP. If I had a SIP address that I could access, I will include it here to offer an example without invading someone else’s privacy.