[HTC One X] Thömy:
Sorry, but I can't find anything in the logcats that tells me the reason. Lots of messages and lots of errors. Most of them are in both logs. So they probably not the cause. This snipped contains the logs where it switches from Bluetooth to the earpiece.

03-18 10:13:32.544 D/AudioRouter(978): onMuteChange: false
03-18 10:13:32.544 D/AudioRouter(978): AudioMode: BLUETOOTH
03-18 10:13:32.544 D/AudioRouter(978): Supported AudioMode: EARPIECE, BLUETOOTH, SPEAKER
03-18 10:13:32.549 I/CallHandlerServiceProxy(978): Updating with new audio mode: BLUETOOTH with mute false
03-18 10:13:32.549 I/InCall (982): CallHandlerService - onAudioModeChange : BLUETOOTH
03-18 10:13:32.549 D/CallHandlerServiceProxy(978): onSupportAudioModeChange: EARPIECE, BLUETOOTH, SPEAKER
03-18 10:13:32.549 I/InCall (982): CallHandlerService - onSupportedAudioModeChange : EARPIECE, BLUETOOTH, SPEAKER
03-18 10:13:32.549 D/BluetoothPhoneService(978): handleMessage: 1
03-18 10:13:32.549 D/BluetoothPhoneService(978): handlePreciseCallStateChange: foreground: ACTIVE background: IDLE ringing: IDLE
03-18 10:13:32.554 E/BluetoothPhoneService(978): Could not get a handle on Connection object for the call
03-18 10:13:32.554 V/BluetoothPhoneService(978): Call state Converted2: IDLE/ACTIVE -> 6
03-18 10:13:32.554 D/BluetoothAdapterService(1104415424)( 2266): getState(): mAdapterProperties: com.android.bluetooth.btservice.AdapterProperties@41d54008
03-18 10:13:32.554 E/HeadsetStateMachine(2266): terminateScoUsingVirtualVoiceCall:No present call to terminate
03-18 10:13:32.564 D/InCall (982): InCallPresenter - Notify com.android.incallui.CallButtonPresenter@41e7de88 of state INCALL
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Updating call UI for call: Call{mCallId=9, mState=ACTIVE, mDisconnectCause=NOT_DISCONNECTED, mCapabilities=83, mConnectTime=1395134012480, mChildCallIds=[], mGatewayNumber=null, mGatewayPackage=null, mIdentification=CallIdentification{mCallId=9, mNumber=xxxx, mNumberPresentation=1, mCnapName=null, mCnapNamePresentation=0}, mForwarded=false, mHeldRemotely=false, mCallDetails= calltype10 domain4 erroinfonull mConfParList multipartyfalse , mCallModifyDetails= calltype10 domain4 erroinfo mConfParList multipartyfalse , mSubscription=0}
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Show hold true
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Enable holdtrue
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Show merge false
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Show swap false
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Show add call true
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Show mute true
03-18 10:13:32.569 D/InCall (982): CallButtonPresenter - Show modify call false
03-18 10:13:32.574 D/InCall (982): CallButtonPresenter - isGeneric: false
03-18 10:13:32.574 D/InCall (982): CallButtonPresenter - mShowManageConference : false
03-18 10:13:32.574 D/InCall (982): CallButtonPresenter - mShowGenericMerge: false
03-18 10:13:32.574 D/InCall (982): InCallPresenter - Notify com.android.incallui.ConferenceManagerPresenter@420e4380 of state INCALL
03-18 10:13:32.574 D/InCall (982): InCallPresenter - Notify com.android.incallui.ProximitySensor@41de4bd8 of state INCALL
03-18 10:13:32.574 D/InCall (982): InCallPresenter - Notify com.android.incallui.StatusBarNotifier@41debd70 of state INCALL
03-18 10:13:32.574 D/InCall (982): StatusBarNotifier - onStateChange
03-18 10:13:32.574 D/InCall (982): StatusBarNotifier - updateNotification
03-18 10:13:32.574 D/InCall (982): StatusBarNotifier - updateInCallNotification(allowFullScreenIntent = false)...
03-18 10:13:32.574 D/InCall (982): StatusBarNotifier - cancelInCall()...
03-18 10:13:32.574 D/InCall (982): - updateInCallNotification: timer cleared
03-18 10:13:32.634 D/InCall (982): CallHandlerService - executeMessage 4
03-18 10:13:32.634 I/InCall (982): CallHandlerService - ON_AUDIO_MODE: BLUETOOTH, muted (false)
03-18 10:13:32.634 D/InCall (982): CallHandlerService - executeMessage 5
03-18 10:13:32.634 I/InCall (982): CallHandlerService - ON_SUPPORTED_AUDIO_MODE: EARPIECE, BLUETOOTH, SPEAKER
03-18 10:13:32.634 D/InCall (982): CallButtonFragment - updateAudioButtons - popup menu mode
03-18 10:13:32.634 V/InCall (982): CallButtonFragment - audioButtonEnabled: true
03-18 10:13:32.634 V/InCall (982): CallButtonFragment - audioButtonChecked: false
03-18 10:13:32.634 V/InCall (982): CallButtonFragment - showMoreIndicator: true
03-18 10:13:32.634 V/InCall (982): CallButtonFragment - showBluetoothIcon: true
03-18 10:13:32.634 V/InCall (982): CallButtonFragment - showSpeakerphoneOnIcon: false
03-18 10:13:32.634 V/InCall (982): CallButtonFragment - showSpeakerphoneOffIcon: false
03-18 10:13:32.634 V/InCall (982): CallButtonFragment - showHandsetIcon: false
03-18 10:13:32.634 D/InCall (982): CallButtonFragment - 'layers' drawable: android.graphics.drawable.LayerDrawable@41b81d58
03-18 10:13:32.864 I/Metrics:BatchTransmitte(29704): QueuePusher.sendBatches - Drained batch queue.; Number of batches sent: 0
03-18 10:13:33.509 E/HeadsetStateMachine(2266): terminateScoUsingVirtualVoiceCall:No present call to terminate
[B]03-18 10:13:33.509 E/bt-btm (2266): btm_sec_disconnected - Clearing Pending flag[/B]
03-18 10:13:33.604 D/BluetoothManager(978): mReceiver: HEADSET_AUDIO_STATE_CHANGED_ACTION
03-18 10:13:33.604 D/BluetoothManager(978): ==> new state: 10
03-18 10:13:33.604 D/AudioRouter(978): onBluetoothIndicationChange false
03-18 10:13:33.609 D/AudioRouter(978): calculateModeFromCurrentState EARPIECE
03-18 10:13:33.609 D/BluetoothManager(978): isBluetoothAvailable()...
03-18 10:13:33.614 D/BluetoothAdapterService(1104415424)( 2266): getState(): mAdapterProperties: com.android.bluetooth.btservice.AdapterProperties@41d54008
03-18 10:13:33.629 D/BluetoothAdapterService(1104415424)( 2266): getState(): mAdapterProperties: com.android.bluetooth.btservice.AdapterProperties@41d54008
03-18 10:13:33.634 D/BluetoothMapService(2266): onReceive
03-18 10:13:33.634 D/BluetoothMapService(2266): onReceive, action android.bluetooth.device.action.ACL_DISCONNECTED
03-18 10:13:33.639 I/BluetoothHeadsetServiceJni(2266): connection_state_callback
03-18 10:13:33.649 D/BluetoothManager(978): state = 2for headset: xx:xx:xx:xx:xx:xx
03-18 10:13:33.649 D/BluetoothManager(978): ==> true
03-18 10:13:33.649 I/AudioRouter(978): Audio mode changing to EARPIECE
03-18 10:13:33.654 D/AudioRouter(978): AudioMode: EARPIECE
03-18 10:13:33.654 D/AudioRouter(978): Supported AudioMode: EARPIECE, BLUETOOTH, SPEAKER
03-18 10:13:33.654 I/CallHandlerServiceProxy(978): Updating with new audio mode: EARPIECE with mute false
03-18 10:13:33.674 D/HeadsetStateMachine(2266): processConnectionEvent state = 0, device = xx:xx:xx:xx:xx:xx
03-18 10:13:33.679 E/HeadsetStateMachine(2266): terminateScoUsingVirtualVoiceCall:No present call to terminate
03-18 10:13:33.679 D/BluetoothAdapterService(2266): getAdapterService(): returning com.android.bluetooth.btservice.AdapterService@41d40ac0
03-18 10:13:33.684 D/BluetoothAdapterService(1104415424)( 2266): Message: 20
03-18 10:13:33.684 D/BluetoothAdapterService(1104415424)( 2266): MESSAGE_PROFILE_CONNECTION_STATE_CHANGED
03-18 10:13:33.689 D/BluetoothAdapterService(1104415424)( 2266): getState(): mAdapterProperties: com.android.bluetooth.btservice.AdapterProperties@41d54008
03-18 10:13:33.694 D/HeadsetStateMachine(2266): device xx:xx:xx:xx:xx:xx is removed in Connected state
03-18 10:13:33.694 D/BluetoothAdapterProperties(2266): CONNECTION_STATE_CHANGE: xx:xx:xx:xx:xx:xx: 2 -> 0
03-18 10:13:33.694 I/InCall (982): CallHandlerService - onAudioModeChange : EARPIECE
03-18 10:13:33.699 D/CallHandlerServiceProxy(978): onSupportAudioModeChange: EARPIECE, BLUETOOTH, SPEAKER
03-18 10:13:33.704 D/BluetoothManager(978): mReceiver: HEADSET_STATE_CHANGED_ACTION
03-18 10:13:33.704 I/InCall (982): CallHandlerService - onSupportedAudioModeChange : EARPIECE, BLUETOOTH, SPEAKER
03-18 10:13:33.704 D/BluetoothManager(978): ==> new state: 0
03-18 10:13:33.704 D/BluetoothPbapService(2266): action: android.bluetooth.device.action.ACL_DISCONNECTED
03-18 10:13:33.709 D/wpa_supplicant(825): RX ctrl_iface - hexdump(len=22): 44 52 49 56 45 52 20 42 54 43 4f 45 58 53 43 41 4e 2d 53 54 4f 50
03-18 10:13:33.709 D/wpa_supplicant(825): wlan0: Control interface command 'DRIVER BTCOEXSCAN-STOP'
03-18 10:13:33.709 D/AudioRouter(978): onBluetoothIndicationChange false
03-18 10:13:33.714 V/BluetoothFtpReceiver(2266): BluetoothFtpReceiver onReceive :android.bluetooth.device.action.ACL_DISCONNECTED
03-18 10:13:33.714 D/AudioRouter(978): calculateModeFromCurrentState EARPIECE
03-18 10:13:33.714 D/BluetoothManager(978): isBluetoothAvailable()...
03-18 10:13:33.714 V/BluetoothFtpReceiver(2266): BluetoothFtpReceiver Start Service
03-18 10:13:33.714 D/BluetoothAdapterService(1104415424)( 2266): getState(): mAdapterProperties: com.android.bluetooth.btservice.AdapterProperties@41d54008
03-18 10:13:33.719 V/BluetoothFtpService(2266): Ftp Service onStartCommand
03-18 10:13:33.719 V/BluetoothFtpService(2266): action: android.bluetooth.device.action.ACL_DISCONNECTED
03-18 10:13:33.719 D/BluetoothManager(978): ==> false