Your phone rings. You answer. The person on the other end sounds like they're calling from the bottom of a pool, and you're saying "what?" four times in a row.
Before you blame your carrier, look at the speaker holes. Seriously, hold the phone up to the light.
The little holes are a lint magnet
Phones pull a lot of daily life into their speaker grilles. Pocket fuzz, backpack crumbs, sunscreen residue, and out here in Escondido, fine trail dust from a weekend hike. It packs in a little at a time, so the sound doesn't drop off a cliff. It just gets dull, week after week, and you turn the volume up without noticing.
The giveaway is when music or a podcast sounds fine in earbuds and flat through the phone itself.
Speaker or mic? Two-minute test
Play a song out loud. Then record a voice memo and play it back.
If the song is muffled, it's the speaker side. If the song sounds fine but your voice on the recording is faint, tinny or gone, the problem is the microphone, and that's a different fix. People mix these up all the time, and "they can't hear me" gets blamed on the other person's phone.
What I'd try first
Take the case off and check the openings line up. Cases drift, especially after a few drops.
Then a soft, dry brush, an unused toothbrush works, with light strokes across the grille. Not a pin. Not a toothpick. Not a blast of compressed air, which can shove dust further in or stress the speaker membrane. And no liquids, ever. If the phone got wet recently, that's a separate story and the dull sound might be moisture, so stop and let me look.
When it's not lint
If the grille is clean and it still sounds like a pillow, a few things could be going on. The speaker itself may be wearing out. The mesh behind the grille might be clogged deeper than a brush can reach. Or there's old water damage nobody noticed. I can usually tell in a few minutes once I open it up, and a speaker swap is a very ordinary repair, not a big scary one.
I come to you, so the phone isn't gone for days. Escondido, San Marcos, Vista, anywhere in North County.
If a brush and a case check don't fix it, send me the model and tell me whether the problem is the speaker or the mic. That one detail saves a lot of guessing.