Voicemail Email Subject only Appended

In Voicemail admin (module version 12.0.27) there is a setting for "email subject" where the admin can set a custom email subject.

Problem is that this only prepends to a hidden, preset email subject.
The effect is this:
If I set the value to
"test"
the email I receive has the subject line:
"test #2 for 2536 #2 in mailbox 2536"

So as admin, I can't really write my own subject line. I can only prepend to a non-custom one.