Don't hardcode ru into the digits filename so that languageprefix can work.

(closes issue #12404)
Reported by: IgorG
Patches:
      voicemail_ru_hardcoded-v1.patch uploaded by IgorG (license 20)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@114027 65c4cc65-6c06-0410-ace0-fbb531ad65f3