Probably akin to pheggos and thus to phemi; to utter a clear sound, i.e. (generally) to proclaim -- speak.
see GREEK phemi
see GREEK pheggos
1. to give out a sound, noise, or cry; used by the Greeks of any sort of sound or voice, whether of man or animal or inanimate object — as of thunder, musical instruments, etc.; (φθέγγεσθαι denotes sound in its relation to the hearer rather than to its cause; the μέγα λαλῶν is a braggart, the μέγα φθεγγόμενος. is a lofty orator; Schmidt, Syn., chapter 1 § 53).
2. to proclaim; to speak, utter: Acts 4:18; ὑπέρογκα, 2 Peter 2:18 (ἄδικα, Wis. 1:8); ὑποζύγιον, ἄφωνον ἐν ἀνθρωπίνῃ φωνή φθεγξάμενον, 2 Peter 2:16. (Compare: ἀποφθέγγομαι.)