Hey Tia,
I understand that you are having a problem with using Siri and that you can only use it with your headset. It sounds like you are have done some good troubleshooting to test the microphone on your iPhone 6 Plus. I would check what you have done with the troubleshooting outlined below. If that does not work, you will want to Contact Apple Support for further assistance.
If your voice is too faint or sounds unclear using iPhone, or iPod touch
https://support.apple.com/en-us/HT203792
Follow these steps, testing after each with Voice Memos.
- Unplug anything plugged into the headset jack.
- If you're on a call and holding your phone next to your ear, speak into the microphone and make sure you don't block it with your fingers or shoulder.
- If you're using a screen protector or case that covers the microphone, remove it. Clear any debris from the microphones.
- Restart your device.
- Update to the latest version of iOS.
If you've tried these steps and people still can't hear you well on calls, contact Apple Support.
Take care.