I trying to update my status to Offline (Skype for Business) , by POSTing
{"availability":"Offline"}
to me/presence url.
The response is: 400 Bad request
<code>BadRequest</code>
<subcode>ParameterValidationFailure</subcode>
<message>Please check what you entered and try again.</message>
<debugInfo />
<parameters>
<property name="availability">Invalid</property>
</parameters>
Same happens with 'Away' status, but statuses like 'Online', 'Busy' work fine.
If you want to change your status to offline, the only solution I know about would be to issue a DELETE request to the application URL specific to that UCWA session. There really isn't an offline-mode for UCWA. The enumeration for Availability includes all the statuses, but sadly not an accurate description of what can/cannot be set via UCWA.
You should be able to set the following statuses:
Offline (as stated above) is set as a result of the application no longer existing.
IdleBusy/IdleOnline are states that the server will set when the inactivity timer is trigger in the Busy/Online states.