Hello jpmalesido,
I'm sorry to hear you are having issues with your iPhone. If you continue to have issues with the receiver audio on your iPhone, you may find the troubleshooting steps outlined in the following article helpful, including contacting Apple Support directly if necessary. In particular, you may want to make sure there is no debris in the headphone jack that would make the iPhone think that the headphones are still plugged in.
If you can't hear a person on a call or voicemail or if the sound quality is poor on iPhone - Apple Support
Sincerely,
- Brenden