Flex offers basic functionality for inbound and outbound calling capabilites by default. You can augment existing calls using the Programmable Voice API. Programmable Voice can also be used, alongside other tools like Studio, to set up custom call flows.
You can pull calls in your Flex account by using the Usage Records API and passing
pstnconnectivity as the Category parameter. This will show your Programmable Voice usage over the PSTN (aka, the actual telephone network.)
Receiving a call is one of the basic interactions with the Flex UI. Other workflows, like making an outbound call or transferring a call, are more complex. The following end user guides explain how these interactions work in the Flex UI in more detail.