As promised in the release notes for the Windows 10 build 14390 notes for PC and Mobile Insiders, a new update is now available for the Voice Recorder app in the Windows Store for Fast ring Insiders on mobile. While minor, the update fixes a bug that would sometimes cause the app to be missing from call recording options.
The update was confirmed to now be available by Windows Insider chief Dona Sarkar on Twitter. Here's a quick recap of what was fixed, from the build 14390 notes:
Voice Recorder does not show up consistently as an option for Call Recording. We heard your feedback and plan to release a Voice Recorder app update via the Store soon that has the fix for this issue.
In any case, if you're a Fast ring Insider on Windows 10 Mobile build 14390, you should now be able to snag the update in the Windows Store.