Looks like it works.. sort of.. I can put a unique in there but it complains that its not in the proper format. so after the ID# I need to put @a.a at the end, and it'll accept it. I went back and checked embedded VM licences, and we have none allocated, none available, none used, etc.. so it appears to not use a license if that field is filled in. Probably only uses a license if the voicemail checkbox is checked in that other tab.
So when we export data, we'll just have to massage that field, and take out the @a.a guess that'll have to do.