Skype for Business error SIP/2.0 503 Service Unavailable

2017-12-11T10:57:31+00:00 March 22nd, 2017|Skype|

I recently worked on a project where the client was attempting to configure a SIP trunk between an Avaya IP-PBX and Skype for Business Server to enable Dial-in Conferencing. All inbound calls were being rejected with a SIP/2.0 503 Service Unavailable error message.
The Windows Event Log showed the calls were being dropped due to an internal server error with no further explanation.
We captured SIP messages using CLS Logger but the SIP messages did not contain any information to explain why the calls were being dropped.
Further investigation discovered that the Mediation, Conferencing Attendant and Response Group Service were not at the correct patch version.

ComponentName                                                                                                        Version
————-                                                                                                                     ——-
Skype for Business Server 2015, Administrative Tools                                       6.0.9319.0
Skype for Business Server 2015, Application Host                                              6.0.9319.0
Skype for Business Server 2015, Audio Test Service                                           6.0.9319.0
Skype for Business Server 2015, Backup Service                                                 6.0.9319.0
Skype for Business Server 2015, Call Park Service                                               6.0.9319.0
Skype for Business Server 2015, Conferencing Announcement Service          6.0.9319.0
Skype for Business Server 2015, Conferencing Attendant                                 6.0.9319.0
Skype for Business Server 2015, Conferencing Server                                       6.0.9319.259
Skype for Business Server 2015, Core Components                                           6.0.9319.259
Skype for Business Server 2015, Core Management Server                              6.0.9319.0
Skype for Business Server 2015, Front End Server                                              6.0.9319.259
Skype for Business Server 2015, Mediation Server                                              6.0.9319.0
Skype for Business Server 2015, Performance Counter Cumulative Update  6.0.9319.259
Skype for Business Server 2015, Reach Fonts                                                       6.0.9319.0
Skype for Business Server 2015, Response Group Service                                 6.0.9319.0
Skype for Business Server 2015, Skype for Business Web Application             6.0.9319.102
Skype for Business Server 2015, Web Components Server                                6.0.9319.259
Skype for Business Server 2015, Web Conferencing Server                               6.0.9319.0
Skype for Business Server 2015, XMPP Translating Gateway                             6.0.9319.0

This happened because Enterprise Voice was deployed after the server had been patched. When a new component is deployed the server must be re-patched to bring that component up to the same level. After running the server update again the versions are all at the correct level

ComponentName                                                                                                        Version
————-                                                                                                                     ——-
Skype for Business Server 2015, Administrative Tools                                       6.0.9319.0
Skype for Business Server 2015, Application Host                                              6.0.9319.0
Skype for Business Server 2015, Audio Test Service                                           6.0.9319.0
Skype for Business Server 2015, Backup Service                                                 6.0.9319.0
Skype for Business Server 2015, Call Park Service                                               6.0.9319.0
Skype for Business Server 2015, Conferencing Announcement Service          6.0.9319.0
Skype for Business Server 2015, Conferencing Attendant                                 6.0.9319.235
Skype for Business Server 2015, Conferencing Server                                       6.0.9319.259
Skype for Business Server 2015, Core Components                                           6.0.9319.259
Skype for Business Server 2015, Core Management Server                              6.0.9319.0
Skype for Business Server 2015, Front End Server                                              6.0.9319.259
Skype for Business Server 2015, Mediation Server                                              6.0.9319.259
Skype for Business Server 2015, Performance Counter Cumulative Update  6.0.9319.259
Skype for Business Server 2015, Reach Fonts                                                       6.0.9319.0
Skype for Business Server 2015, Response Group Service                                 6.0.9319.259
Skype for Business Server 2015, Skype for Business Web Application             6.0.9319.102
Skype for Business Server 2015, Web Components Server                                6.0.9319.259
Skype for Business Server 2015, Web Conferencing Server                               6.0.9319.0
Skype for Business Server 2015, XMPP Translating Gateway                             6.0.9319.0

Inbound calling now works as expected.