Transfer to agent or queue using Salesforce Voice Toolkit API addParticipant

Attempting to add an Agent ARN or Queue ARN through the Add Participant action results in a CAN_NOT_ADD_PARTICIPANT error with the message: "Invalid agent or contact state.

Steps to Reproduce

1. Establish an active voice call.
2. Enter a valid Agent ARN or Queue ARN in the Add Participant field.
3. Click Add Participant.

Observed Behavior

The request fails and the following error is logged:

SCV dispatched error CAN_NOT_ADD_PARTICIPANT for eventType ADD_PARTICIPANT

{

  "type": "InvalidStateException",

  "message": "Cannot create new connection by endpoint ARN: <Agent ARN>: Invalid agent or contact state."

}

Expected Behavior

The specified agent or queue should be added to the active call successfully.