Do the ones who don't hear you hear you if you switch to speakerphone?
If it's only some, it's a carrier problem. If they all hear you on speakerphone, but not on the regular phone, it's either dist and dirt crammed into the mic hole (it's pretty tiny, so a fingerful of dust is a pretty solid sound insulator) [clean it out with a thin sharp wooden toothpick, a strong light and, depending on your eyes, a magnifying glass] or it's a bad mic.
(The video recording mic is usually the speakerphone mic, not the phone mic.)