The recording is done by the conferencing module, and only starts at the point the parties are connected to the conference.
This happens after the client hears “Connecting you to the interpreter. To call an additional party, press 9”, and at the same time the interpreter hears “Please hold, while we connect you to the caller.”
If the caller hangs up (either deliberately, or their call was lost), during that introductory announcement, before they were actually conferenced to the interpreter, there will not be a recording. However, the system will consider it a successful (and chargeable) call because the interpreter had answered and accepted the call.