Board Thread:General Discussion/@comment-24973452-20160905181416/@comment-27701762-20160905181923

I tend to agree that Mettaton was about to say "he," though there's not much to go on, and Mettaton's diaries refer to Napstablook as "they."

Theoretically Mettaton could have been about to say "hey" or "hello" or something else like that to double-check if there was someone at the other end of the line (something not too uncommon when you are abruptly hung up on).