Surely it would have been easier if the verse had said, “And the Lord called to Moses”?
Yet the text goes on, “And the Lord spoke to him…”, so why leave out the name of God in the first phrase?
Possibly what happened was that Moses became aware that someone was calling him and only then recognised the voice and realised that he was being addressed by God.
Other people, maybe all of us, have the same experience. We are preoccupied with something and become aware, at first dimly, that someone is talking to us. Then we recognise the voice and come back to reality.
Someone I visited in hospital told me that this happened to him after an operation. He was in the recovery room and had not yet completely come round after the anaesthetic but he dimly heard someone calling him and he struggled to emerge from his semi-consciousness to find that the nursing staff were trying to rouse him.
In the case of Moses – long before anaesthetics – it was not that he was coming round from an operation but that he may have been concentrating on how to handle a particular leadership task. Or he may have been meditating and almost oblivious to his surroundings.
When God knew Moses was responding to the call, He could then continue with the message of the moment.