delete voicemail only if it isn't linked
refactor voicemail. Only delete voicemail, not links
implement link checker in voicemail dao
add capital letter at beginning of error message
make voicemail error message more explicit
Merge branch '4686_delete_voicemail'

* 4686_delete_voicemail:
  make voicemail error message more explicit
  add capital letter at beginning of error message
  implement link checker in voicemail dao
  refactor voicemail. Only delete voicemail, not links
  delete voicemail only if it isn't linked