‘And I will encamp against you in a circle (literally ‘like a circular object' surrounding the city),
And will lay siege against you with a fort,
And I will raise siegeworks against you.
And you will be brought down,
And you will speak from the ground,
And your speech will be low out of the dust,
And your voice will be as of one who has a familiar spirit, out of the ground,
And your speech will whisper (chirp), out of the dust.'
There is a direct contrast between David encamping in the city in Isaiah 29:1 with God encamping against it here. The tables have been turned.
Some however translate Isaiah 29:1 as ‘against which David encamped', a possible translation, and see this as indicating parallel action (Hebrew prepositions are not always as clear as we would like). David had once encamped against it when it was a pagan city, and now God was doing the same for the same reason. The basic idea is the same.
Yahweh (initially through the king of Assyria) intends to lay siege to Jerusalem, this false mountain of God, raising siegeworks against it and humbling it. It will be brought so low that its voice will, as it were, seem to come out of the ground, from the very dust. They will be utterly humbled. And they will be in such a state that their prayers will be like the whispered words of a medium, like the chirp of an enchanter who seeks to the dead. (For the idea of prayer seen as such chirping/whispering in distress see Isaiah 26:16). Again there is a mixing of the contrast between the idea of God with ‘the gods', especially underworld gods, as the people's prayers to God seem almost similar to appeals made to these ‘gods'.