Hey there Stowe Away,
It sounds like your headphones that came with the phone only work on the right side. I recommend these steps from the article named:
Troubleshooting iPhone, iPad, and iPod touch accessories
3. Disconnect and Restart
Isolate the issue to the accessory, if possible. Or, if available, try to reproduce the issue with another identical or similar accessory.
6. Restore iPhone, iPad, or iPod touch
Restore iPhone, iPad, or iPod touch to its factory default state. You should rarely need to use this step with an accessory issue. If the issue only affects a third-party accessory, contact the manufacturer before restoring the device.
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All the best,