Why can't someone hear me but I can hear them

It could be as simple as a plugged mic port. If you know where the mic port is located try cleaning it out with the brissels of a brush and then blowing it out. If that does not work try resetting the phone. And then if that doesn't work it would be a hardware problem.

If this occurs with more than one person, you most likely have an issue with your device's microphone. If this only happens with one person however, they probably have an issue with their speaker. Try speaking while the phone is on speaker, and check to see if anything is obstructing the bottom of the device. Good luck, let me know if you need more help!

Is Sprint your cell phone provider? Sprint sometimes has the problem you have described. That's one of the reasons I switched from Sprint to Verizon.

Or maybe the person you are talking to has Sprint. If all such people have Sprint, or if you have Sprint, I suspect that Sprint is the source of the problem.