In Amazon Connect, identify the third party call disconnect on conference using amazon-connect-streams

Can you describe more of the exact use case? Is this transfer initiated by the agent and they complete the transfer so the caller is now waiting/talking to an external phone number that the agent cannot control? or is this done through a "Transfer to Phone Number" block while caller waiting in Queue?

And when you mention notification, who is it notifying? if the agent competed the transfer, they would most likely be handling another call.

One way to get "notify" is through Contact Event Stream using EventBridge, you will get an event for all contacts' events including Disconnect. Depending on how you construct a list of call that you want to notify, Events from EventBridge can trigger an S3, Lambda, or other mechanism for you to do various customizations.