For anyone dealing with agents not being able to transfer calls, there seems to be an issue with the agent not passing in the forwarding number to the tool function. The messages section in the calls logs shows this:

"toolCalls": [
    {
      "id": "call_UD9hqpCH9sGQHZNjGnXzo8kX",
      "type": "function",
      "function": {
        "name": "transferCall",
        "arguments": "{}"
      }
    }
  ],