It’s important to note that VoIP is a phone service — like dial tone — to which businesses subscribe, just as they would with a time-division multiplexing (TDM) service for PSTN-based telephony. With the advent of VoIP came the IP PBX, which has now become the successor for premises-based phone systems. Similarly, SIP trunking has emerged as the successor to T1 for the trunking service to support enterprise telephony.
With the relationship between VoIP and SIP explained, the similarities with SIP trunking become clearer. While SIP is one of the protocols used for VoIP, SIP trunking provides the actual connection between a telephony endpoint, such as an IP PBX or a SIP phone, and a data network over which a VoIP call will travel. As such, they are highly complementary, especially for enterprises looking to migrate from legacy phone systems.