Jamieson, Fausset and Brown on Numbers 7:89

89. And when Moses was gone into the tabernacle of the congregation to speak with him--As a king gives private audience to his minister, so special license was granted to Moses, who, though not a priest, was admitted into the sanctuary to receive instructions from his heavenly King as occasion demanded.

then he heard the voice of one speaking to him--Though standing on the outer side of the veil, he could distinctly hear it, and the mention of this circumstance is important as the fulfilment, at the dedication of the tabernacle, of a special promise made by the Lord Christ Himself, the Angel of the Covenant, commanding its erection (Ex 25:22). It was the reward of Moses' zeal and obedience; and, in like manner, to all who love Him and keep His commandments He will manifest Himself (Joh 14:21).