Phone Reason for Out of Service Codes

Examine jabber.log or Debug Display on a phone's web server to look for entries after a disconnect occurred.
Then refer to this list, which is scraped from CUCM Serviceability > Alarm > Definition > CallManager Alarm Catalog > Phone > Find > LastOutOfServiceInformation.
Note that CUCM 11.5 added some more reason codes but the alarm catalogue hasn't been updated, there's a bug listing for this: CSCvb63000

10   --  TCPtimedOut - The TCP connection to the Cisco Unified Communication Manager experienced a timeout error
12   --  TCPucmResetConnection - The Cisco Unified Communication Manager reset the TCP connection
13   --  TCPucmAbortedConnection - The Cisco Unified Communication Manager aborted the TCP connection
14   --  TCPucmClosedConnection - The Cisco Unified Communication Manager closed the TCP connection
15   --  SCCPKeepAliveFailure - The device closed the connection due to a SCCP KeepAlive failure
16   --  TCPdeviceLostIPAddress - The connection closed due to the IP address being lost.  This may be due to the DHCP Lease expiring or the detection of IP address duplication. Check that the DHCP Server is online and that no duplication has been reported by the DHCP Server
17   --  TCPDeviceRegsistrationTimedOut - The device closed the TCP connection due to a registration timeout
18   --  TCPclosedConnectHighPriorityUcm - The device closed the TCP connection in order to reconnect to a higher priority Cisco Unified CM
20   --  TCPclosedUserInitiatedReset - The device closed the TCP connection due to a user initiated reset
22   --  TCPclosedUcmInitiatedReset - The device closed the TCP connection due to a reset command from the Cisco Unified CM
23   --  TCPclosedUcmInitiatedRestart - The device closed the TCP connection due to a restart command from the Cisco Unified CM
24   --  TCPClosedRegistrationReject - The device closed the TCP connection due to receiving a registration rejection from the Cisco Unified CM
25   --  RegistrationSuccessful - The device has initialized and is unaware of any previous connection to the Cisco Unified CM
26   --  TCPclosedVlanChange - The device closed the TCP connection due to reconfiguration of IP on a new Voice VLAN
27   --  TCPclosedPowerSavePlus - The device closed the TCP connection in order to enter Power Save Plus mode
100  --  ConfigVersionMismatch - The device detected a version stamp mismatch during registration Cisco Unified CM
104  --  TCPclosedApplyConfig - The device closed the TCP connection to restart triggered internally by the device to apply the configuration changes
105  --  TCPclosedDeviceRestart - The device closed the TCP connection due to a restart triggered internally by the device because device failed to download the configuration or dial plan file
106  --  TCPsecureConnectionFailed - The device failed to setup a secure TCP connection with Cisco Unified CM
107  --  TCPclosedDeviceReset - The device closed the TCP connection to set the inactive partition as active partition, then reset, and come up from the new active partition
108  --  VpnConnectionLost - The device could not register to Unified CM because VPN connectivity was lost
200  --  ClientApplicationClosed - The device was unregistered because the client application was closed
201  --  OsInStandbyMode - The device was unregistered because the OS was put in standby mode
202  --  OsInHibernateMode - The device was unregistered because the OS was put in hibernate mode
203  --  OsInShutdownMode - The device was unregistered because the OS was shut down
204  --  ClientApplicationAbort - The device was unregistered because the client application crashed
205  --  DeviceUnregNoCleanupTime - The device was unregistered in the previous session because the system did not allow sufficient time for cleanup
206  --  DeviceUnregOnSwitchingToDeskphone - The device was unregistered because the client requested to switch from softphone to deskphone control
207  --  DeviceUnregOnSwitchingToSoftphone - The device is being registered because the client requested to switch from deskphone control to softphone
208  --  DeviceUnregOnNetworkChanged - The device is being unregistered because the client detected a change of network
209  --  DeviceUnregExceededRegCount - The device is being unregistered because the device has exceeded the maximum number of concurrent registrations
210  --  DeviceUnregExceededLoginCount - The device is being unregistered because the client has exceeded the maximum number of concurrent logons