When restoring a course backup and choosing "Existing course, deleting first" Moodle fails with the error:

Function voicetools_delete_instance() doesn't exist!

An error occurred while deleting some of the course contents.

This has been replicated many times, and it doesn't matter whether the target course has any or no resources/acivities in it.

Hiding the Voice Tools module does not solve the problem, though I haven't tried to delete it yet (need to mirror the server first).