Callers say they can hardly hear me when talking on my iPhone 7
The first step is to check Bluetooth, headset speakerphone and normal receiver function. 

If the volume is bad on all, look to software restore or work with the carrier. 

If not and assuming you're not having problems with recording videos etc, and this is only happening during a normal phone call (that is, not on speakerphone).

If so, one test you can do is to open the Voice Memos app and record yourself speaking into it. I would record maybe 10 seconds of yourself speaking and then play it back. You should be able to hear yourself clearly. 

If not, this could indicate a hardware problem, but before jumping to that conclusion there are other things to try:


restart your iPhone
Remove any case and/or screen protector you're using
Unplug any accessories you have connected to the headphone jack 
Remove any dirt/dust that could be blocking the microphone 


If none of these help, I would contact Apple.