Examples beat adjectives
“Friendly and professional” is a mood, not an instruction. A better brief includes three real examples, the sentence patterns you use often, and the phrases your customers use themselves.
Show the difference between a short answer and a long explanation. Tell the agent when a direct sentence is better than a clever one. Voice becomes easier to follow when it has something concrete to imitate.
Write down the edges
Every brand has subjects it should handle carefully, claims it cannot support, and words it never uses. Those boundaries matter just as much as your preferred tone.
Include how the brand should describe its relationship to the product. A transparent affiliation line is part of the voice, not a legal footnote added at the end.
Review for recognizability
Read a draft without looking at the agent name. Would a teammate know which brand wrote it? If not, add a better example or remove a rule that is too abstract.
A short voice brief that your team can edit is more useful than a long document no one opens.