Worth noting, the call transfer code in Vapi is quite buggy, you will likely need to do some updating to your prompt to get it to work smoothly. Lastly, the actual call transfer tool that is provided to the LLM has a description that is generated by Vapi and not visible to you. It REALLY looks like this (in case helpful)
Use this function to transfer the call. Only use it when instructions explicitly ask you to use the transferCall function. DO NOT call this function unless you are instructed to do so.\n\nHere are the destinations you can transfer the call to:\n 1. [YOUR TRANSFER NUMBER]. When: [THE DESCRIPTION YOU WROTE]