SIP Trunking enables an enterprise to connect their on-premise voice network implemented by Microsoft Office Communications Server 2007 R2 to the Public Switched Telephony Network (PSTN) directly over SIP. SIP Trunking services are offerings from IP Telephony Service Provider partners that offer PSTN origination and termination, using the SIP protocol.
Sprint now joins the other four partners whose SIP Trunking Services have been independently qualified to meet UC Open Interoperability Program (UC OIP) requirements. Cool!
Source: Voice Specialized Partner Sales and Technical Resources: January 2010