Dr. Van Groningen argues that Isaiah wrote Isaiah 40-66.
Gerard Van Groningen, "The Messiah in the Book of Isaiah, 3: The Servant-Son," Messianic Revelation in the Old Testament volume 2 (Wipf and Stock Publishers: Eugene OR, 1997), 579-580
The voice calling is to be distinguished from the earlier spokesman, who received the command to comfort Yahweh's people and to speak to the heart (tenderly) of Jerusalem by proclaiming forgiveness (vv. 1-2). This one also responded to the voice (v. 6) asking what to "cry out," and he was told to proclaim that in the midst of change among the peoples of the world, the word of Yahweh stands forever (v. 8). The latter one is undoubtedly the prophet Isaiah who received, as it were, a second call (cf. esp. 40:9) to bring a prophetic message to the covenant people and to mankind. His message is summarized in 40:6b-11. The prophet, often speaking in such a way that it was Yahweh's own message, refers to himself as the messenger of good tidings (41:27). And he specifically says he has been sent by the sovereign Lord and equipped with his Spirit to call the people of Yahweh back from bondage (48:16-21). Beyond a doubt, then, the prophetic proclamation of Isaiah is to be considered predominant. As in the first part of his prophecy, he is Yahweh's spokesman; he occupies the prophetic office and carries out its designated task. As such he continues to stand and function in the prophetic, Mosaic line, which gained its fullest expression and culmination in the promised Messiah, even Jesus Christ.
The voice of which Isaiah speaks, to which he responds, is not identified. It serves as a spokesman for Yahweh. It also functions on behalf of Isaiah the prophet. Thus, the voice fulfills a prophetic function in speaking on behalf of Yahweh, but it also serves this function by calling attention to it, reminding people of its task, and outlining the message to be proclaimed by the prophet. It an now be readily seen that what the voice is for Isaiah, John the Baptist later is for the greatest prophet, Jesus Christ. So, as Isaiah functioned as a predecessor and type of Christ, so the voice functioned in regard to John the Baptist.