how to use the coller number (event.From) for http request in Studio Flow?

I´m developing an IVR that at some point needs to run a function. The function send http request to a service provider to
make payment for a product selected by the user. The client is identified by his phone number.
Since i have to configure the twilio phone number to Studio Flow (not webhook), how to get dynamically the caller phone number and send it in http request?