SIP Protocol is the emerging standard for VOIP trunk service using Session Initiation Protocol (SIP) to set-up voice calls over public or private IP networks. SIP trunking service is used by businesses to provide local and long distance voice service as well as other applications. SIP trunk service provides increased efficiency through an IP PBX phone system by utilizing IP network access for SIP voice calls, without the need for regular phone lines. SIP VOIP service is very flexible and can work with numerous protocols and scale quickly. SIP trunking providers also offer non-native variations of SIP VOIP service including SIP PRI and SIP T1 handoffs. SIP VOIP services are also known as SIP Trunk Services, SIP Trunking Services, SIP Lines, VOIP PRI, VOIP T1, Hosted VOIP PBX, and Hosted IP PBX.