Error 0x00EF00B7

Value: 15663287 | 0x00EF00B7 | 15663287

What does it mean ?

RTC_E_STATUS_SESSION_PROGRESS  
Session Progress
Value: 183 | 0x00B7 | 0b0000000010110111

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