After the tracing and testing sessions, it was found that Issue is with Equinox client itself, as we did not observe the issue with Old client AVAYA Communicator.
Logs found from Equinox client in bad case:
2017-02-03 14:43:39.915 [INFO] - [CSDK] [Native thread: 13344]: CWebRTCAudioEngine[0x138CC9C0]::StartPlayingFileInChannel : file:'ringback.wav' channelId:0 loop:1 durationMs:0 gain:255 speaker: 1 callback:1
2017-02-03 14:43:39.915 [ERROR] - [CSDK] [Native thread: 13940]: ERROR ; ( 9:13:39:854 | 0) FILE: 2 1025; 13344; Could not open input file C:\Program Files (x86)\Avaya\Avaya Equinox\Resources\Audio/ringback.wav
2017-02-03 14:43:39.915 [WARNING] - [CSDK] [Native thread: 13940]: WARNING ; ( 9:13:39:854 | 0) VOICE: 2 1025; 13344; FilePlayerImpl::StartPlayingFile() failed to initialize file C:\Program Files (x86)\Avaya\Avaya Equinox\Resources\Audio/ringback.wav playout.
2017-02-03 14:43:39.915 [ERROR] - [CSDK] [Native thread: 13940]: ERROR ; ( 9:13:39:855 | 1) VOICE: 2 99; 13344; StartPlayingFile() failed to start file playout, error = 10016
2017-02-03 14:43:39.915 [ERROR] - [CSDK] [Native thread: 13344]: CWebRTCAudioEngine[0x138CC9C0]::StartPlayingFileInChannel : StartPlayingFileLocally[ FAIL ] [-1], hPlayback: 3145777, line = 2241
2017-02-03 14:43:39.915 [WARNING] - [CSDK] [Native thread: 13940]: WARNING ; ( 9:13:39:855 | 0) FILE: 2 1025; 13344; playing is not active!
2017-02-03 16:12:42.502 [INFO] - [CSDK] [Native thread: 8860]: CWebRTCAudioEngine::CreateFilePlayCall : Started!
2017-02-03 16:12:42.502 [INFO] - [CSDK] [Native thread: 8860]: CWebRTCAudioEngine::StartPlayingFileInChannel : file:'ringback.wav' channelId:0 loop:1 durationMs:0 gain:255 speaker: 1 callback:1
2017-02-03 16:12:42.502 [INFO] - [CSDK] [Native thread: 8860]: CWebRTCAudioEngine::RegisterFilePlayInfo : hPlayback: 197633, Channel: 0, Loop: 1, DurationMs: 0, Gain: 255, Speaker: 1, callback: 1, line: 1731
2017-02-03 16:12:42.502 [INFO] - [CSDK] [Native thread: 8860]: CWebRTCAudioEngine::StartPlayingFileInChannel : StartPlayingFileLocally, hPlayback: 197633
SIP/2.0 480 Temporarily Unavailable
P-Location: