Appointed prophets. Sanballat has little esteem for the high calling of a prophet.
He had found so-called prophets ready to cooperate with him against Nehemiah, for pay (see verses 12, 14), and he apparently thought of all prophets as being mercenary like his own (see Amos 7:12). Possibly also Sanballat had had access to prophecies such as those of Zechariah, and either misunderstood or purposely misconstrued them (see Zechariah 1:16; 2:5; 6:11; 9:9, 10; 12:9; 14:9; etc.).
Take counsel together. By sending heavy accusations against him in an “open letter,” that is, one that might be read by everyone, Sanballat thought Nehemiah would seek to clear himself from suspicion by joining in the proposed interview.