Both your examples seem awkward to me.
Unless your POV character hears Trevor's voice before they see him he should be present in the scene before he speaks, and consequently also described before he speaks.
And since he's the POV character's best friend, "I" should notice him even if it's in a big crowd. It's very seldom a best friend would enter a scene dialog first.
Let's say the POV character enters a room where Trevor is sitting on a couch. Then you describe him when the character sees him.
You can (read should) also spread his description out in the scene.
The balance you need to keep an eye on is between info-dumping (just making a shopping list of character features) and jarring the reader with details later on that collides with their internal image of the character or the place or object being described.
But you should also show Trevor's description rather than telling it.
A few rules of thumb when doing first-person singular "I" POV:
- Never describe things your POV character can't see, hear, etc
- Avoid describing things one can assume your POV character knows and wouldn't think of
For instance, "Trevor is my best friend"
You could tell the reader "my best friend Trevor" but it would break the rules above. We seldom think, my best friend, when we meet our best friends, we just start interacting with them.
So, it's more natural if you show the reader with dialog and actions that he and the POV character are best friends.
"Hey Trevor," I said and gave him a quick hug. "You missed basketball yesterday." I thumped my fist to his shoulder. "What's up with that?"
This tells us they are friends, but not necessarily best friends. It's probably going to be ok until something happens that makes the status of the relationship important. Perhaps Trevor does something a best friend shouldn't do:
"We're supposed to be best friends," I said. "What the hell, Trevor?"
Or you show it to the reader by Trevor and "I" doing things together or how they talk with each other and what they talk about. You know, show them being best friends.
His black hair.
This one should also be done with action if possible, maybe something like:
Trevor scratched his black hair. "Sorry about that..."
And his stature.
You could show it by having your POV character talking up to him, or maybe he's leaning back on that couch stretching out his legs so "I" have to step over them to sit on the couch or something similar. "I" might notice him stretched out on the sofa being tall as a beanstalk or something similar... Like the POV character notices it or is a bit surprised by the image even though he's seen Trevor and his stature many times before.
"Yeah," I said, "we could have needed your tall ass there yesterday. Now they wiped the floor with us."