The Collect step
To ask a free-text question, add a Collect step ("Ask the customer and save the answer"). It has three fields: Question to ask, Save answer as (the variable name), and Expected format. On the canvas the same node is called Collect Input.
Save and reuse the answer
The flow sends your question, then saves the customer's next reply under the name in Save answer as. Later steps can use it: drop it into a message with {$workflow_var:key}, or branch on it. Spaces and punctuation in the name are turned into underscores as you type.
Expected format
The Steps view offers three formats: Anything, Email address, and Phone number. If a reply does not fit the chosen format, the flow re-asks the same question a couple of times, then accepts whatever the customer typed and moves on, so nobody gets stuck. The canvas view adds an advanced regex format.