The phone book in the TCH with Nav allows you to add more than one phone number per person. For example a Home, Work, and Cell Phone number. However, you can only add a single voice tag per person in the phone book. Am I correct about this or am I just doing something wrong? Couldn't find anything about it in the owners manual or via a search in this forum.
Does anyone know?

You are correct. If you want a voice tag for each number (home, office, cell) then you need to make each number a separate entry.