Firstly try increasing the volume with the buttons during an actual live call.
If that does not help you could try a forced restart.
Force restart iPhone – Apple Support (UK)
If no better you could Backup, restore to factory, maybe set up without restoring backup and test at that point,, then restore from the backup:-
(If the phone needs attention you will have to do this anyway)
If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails, or you need to ditch the backup and start as new.
See here for the resources you need. Allow a couple of hours:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Otherwise arrange a repair for suspected hardware failure, however bearing in mind the age of this iPhone, the cost of repair (ie a replacement iPhone 6SPlus) would not be cost effective and you would be better spending th cash on a new SE.
Click here > iPhone Repair & Service - Apple Support
Or
Click here > Contact - Official Apple Support
Anything other than Screen or Battery may be a replacement phone and the price is in the article under "Other damage"
See also: Get your iPhone, iPad, or iPod touch ready for service - Apple Support