Hi Jose I am at the restaurant means you have just arrrived.I am in the restaurant means you are inside,sitting down.So both are correct,depending on what you want to say.

Both are fine. You might say "I'm AT the restaurant" if you were standing outside the restaurant talking on the phone to your friend. Once you go inside, you would more likely say "I'm IN the restaurant." You would not say "I'm IN the restaurant" if you were standing outside.

'I am at the restaurant' locates you in the town. You are at the restaurant, not the train station, for example. I am in the restaurant' locates you in a smaller space. For example you are in (inside) the restaurant, not in the restaurant's garden. Or perhaps you are on holiday in a hotel and you are in the hotel's restaurant, not the spa centre.

"In" means inside whereas "at" indicates location. Both are acceptable, but mean different things. "At" is more common. There is no other significance to the words. For some word combinations (collocations), we only use "at" and some we only use "on." You can memorize each one as you learn it by writing it in an original sentence. For example, we say "at home" but not "in home." We say "in the room" but not "at the room."

I'm @ the restaurant. Then, to clarify if you are @ the front door, or in a he parking lot, or out in the patio, or inside the air condtioning, you could tell your friend more.