If you're using Twilio as your telephony provider, you can forward the calls and also voice out the summary message to the agent. But if you're using some other telephony provider, you'll need to use an alternative like SMS (which we've already mentioned) or SIP headers.
Now, let's talk about your SMS part. When you forward the call, you can use the Transfer Destination Request before forwarding. WAPI will ask you for the destination to be the call to be transferred, and this time on your server URL, you can trigger the SMS to your agent saying "Hey, this is the call being done till now, and this is the summary of it." This way, your agent will have a summary before you transfer back the decision before you return the response to the transfer destination request to VAPI.