The differences between VoIP and SIP.
VoIP calls are sent via SIP Trunks. VoIP and SIP work in conjunction with each other! Your telecoms supplier provides the SIP trunks as this is how all your VoIP calls, fax and data are transited.
You cannot make a VoIP call or send a fax to another phone line without SIP trunks. Let’s say you’re in your office; when you make a call via your VoIP phone (by the way VoIP/SIP phones are the same), when you make that call, it’s sent via a SIP account, not a VoIP account. The call is sent over a SIP trunk not VoIP. For example, SIP is like your old PSTN phone line and the VoIP phone is like your old PSTN phone.
IP to IP is not (Vo)IP.trunking. It’s simply an IP (Internet Protocol) network. Of course you can make (Vo)IP to IP only calls with IP phones via your internet connection or local computer network. All IP phones support (Vo)IP/SIP protocols.