Swap Hold call with Active call FAIL.

ISSUE:

In application when swapping the active call with hold call gives the error LINEERR_OPERATIONUNAVAIL. What are the possible reasons to get this error? 

SOLUTION:

Swap-Hold/Setup Transfer

Note: Cisco Unified TSP 4.0 and later do not support this.

The CCiscoLineDevSpecificSwapHoldSetupTransfer class gets used to perform a SetupTransfer between a call that is in CONNECTED state and a call that is in the ONHOLD state. This function changes the state of the connected call to ONHOLDPENDTRANSFER state and the ONHOLD call to CONNECTED state. This allows a CompleteTransfer to be performed on the two calls. In Cisco Unified TSP 4.0 and later, the TSP allows applications to use lineCompleteTransfer() to transfer the calls without having to use the CCiscoLineDevSpecificSwapHoldSetupTransfer function. Therefore, this function returns LINEERR_OPERATIONUNAVAIL in Cisco Unified TSP 4.0 and beyond.

Swap Between Held and Active Calls on Multiple Lines

If your phone supports multiple lines, use line buttons to swap between held and active calls.

For example, if you have a held call on Line 1 and an active call on Line 2, press the Line 1 button to make the Line 1
call active (resume it from hold) and to automatically put the Line 2 call on hold.

This is the feature you can see on  section calling features that is supported by Cisco Unified IP Phone 6921, 6941, 6945, and 6961.