Answer: CODESWITCH

Codeswitch refers to the act of alternating between two or more languages or dialects within a conversation or social setting. It can also refer to adjusting one's language or speech style depending on the situation or audience. The clue "Go into one's 'phone voice'" suggests adapting one's tone or manner of speaking to sound more professional or formal during a phone conversation.

Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: Go into one's "phone voice," say. Explanations are generated by AI and may be inaccurate.