When playing your custom prompt, have you tried using the, "Play Audio on Silence action", which has the capability to detect barge-in to stop and replay the message again after the specified amount of silence is detected? That would help to have your prompt interrupted and restarted to eventually have it play after the beep for cases where the call is being transferred to the outbound call flow prior to the beep. This option and similar notes can be referenced in step 7 at, "Create a call analysis response", where this would be used in conjunction with Beep Detection on the Call Analysis Response.
For the system message you referenced, "This call is going to voicemail..." played before your custom prompt, are you seeing this message coming into the system (as in, this is the voicemail of the dialed number that was reached with Machine detected just before this message) or is it being played out of the system to the number that was dialed? My above answer assumes that this message is coming into the system, your custom play starts somewhere during that message, and then the actual beep comes, which is why your custom play is getting cut off.
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Jason McDowell
Software Development Engineering Manager
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