At the beginning of this Parsha, we find that Hashem speaks to Moshe from the Ohel Moed, the Tent of Meeting (ויקרא א':א'). The Midrash in VaYikra Rabbah (פרשה א' סימן י"א) states that Hashem spoke to Moshe specifically from that place because the walls of the tent cut off the voice of Hashem so that the sound would not go outside, because if it did, people would be terribly frightened by it. The Midrash (שם סימן י"ב) then adds that before the Mishkan was built, there was prophecy among non-Jews as well, but once Hashem started to speak to Moshe in the Ohel Moed, prophecy departed from the non-Jews. The Midrash (שם סימן י"ג) goes on to say that we can see a major difference between the prophets of the Jews and the prophets of the non-Jews. To the prophets of the Jews, He revealed Himself more fully. This is proven by the fact that in our Posuk (שם), Hashem speaks to Moshe using the word "ויקרא," "and He called," while when He speaks to the non-Jewish prophet Billam, the word "ויקר," "and He met," which is a less complete form, is used ( כ"ג:ט"זבמדבר).
Finally, the Midrash (שם) notes that regarding the non-Jewish prophets, Hashem spoke to them only at night, but to the Jewish prophets He spoke during the day as well. Moshe could be called to the tent of Meeting at any time of day or night and would have to respond; this is one of the things that makes Moshe so great, because He saw Hashem clearly and had to be ready to speak with Him at all times.