"Integra Telecom has lab-certified the Fonality trixbox Community Edition (CE) IP PBX system to operate with its SIP Trunking service. The certification helps ensure interoperability and seamless operation between trixbox CE and Integra solutions" - see how specific that is? That's the difference between SIP trunking and PRI. PRI has 2 configurations on the switch. The NI-2 protocol was the primary configuration for RAS boxes and PRI cards on PBX boxes. National ISDN 2 is a standard.
SIP is made up of about 30 RFC's (or opinions if you will). So each carrier configures SIP differently. That's why for SIP trunks inter-operability is so important! Down to the model number.
