Error 0x80EF01F7

Value: -2131820041 | 0x80EF01F7 | 2163147255

What does it mean ?

RTC_E_STATUS_SERVER_SERVICE_UNAVAILABLE  
Service Unavailable
Value: 503 | 0x01F7 | 0b0000000111110111

Where does it come from ?

FACILITY_SIP_STATUS_CODE  
SIP - Session Initiation Protocol (VoIP)
Value: 239 | 0x0EF | 0b11101111

Other Errors for FACILITY_SIP_STATUS_CODE