The documentation originally stated that for Transfer_Req that agent instrument was optional. It is required.

721916: May 25 11:47:39.272 EDT %MIVR-ICD_CTI-7-UNKutboundMessageprocessor : sending msg
: { length=-1
type=CONTROL_FAILURE_CONF,invokeId=14,failureCode=CF_GENERIC_UNSPECIFIED,errorCode=88006,
text=activeConnectionDeviceId(Transfer Controller) null not associated with
rmcmContact(Held Call) identified by heldCallId 16777379 to socket:
Sockethttp://addr= 10.10.10.216 ,port=62384,localport=42027 }

In agentInstrument you should give the Directory Number of
the agent who is transferring the call.

721913: May 25 11:47:39.272 EDT %MIVR-ICD_CTI-7-UNK:ClientConnMgr:
Processing msg on
socket:Sockethttp://addr= 10.10.10.216 ,port=62384,localport=42027 Msg is
{length=54 type=TRANSFER_CALL_REQ,invokeId=14, peripheraId:1,
activeConnectionCallId:16777381, heldConnectionCallId:16777379,
activeConnectionDeviceIdType: 0, heldConnectionDeviceIdType: 0,
callPlacementType : 0, callManagerType: 0, alertRings: 0, callOption: 0,
facilityType: 0, answeringMachine: 0, priority: false, postRoute: false,
_numNamedVariables:0, _numNamedArrays:0, activeConnectionDeviceId: 2002,
heldConnectionDeviceId: 2002, agentInstrument: null, dialedNumber: null,
userToUserInfo:null, callVar1: null, callVar2: null, callVar3: null,
callVar4: null, callVar5: null, callVar6: null, callVar7: null, callVar8:
null, callVar9: null, callVar10: null, callWrapupData: null, facilityCode:
null, authorizationCode: null, accountCode: null }

note that agent instrument is null in the above example and should instead be the directory number of the agent performing the transfer.