Q: i phone 6s plus

Hi margaretfromgrimsby,

Thanks for using the Apple Support Communities!

I understand that the volume from the receiver on your iPhone isn’t as loud as you would expect it to be with the volume turned all the way up. To begin troubleshooting this situation, I recommend reading over and working through the suggested steps in the following article. 

If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone - Apple Support

You also mention that this same issue has happened with a previous iPhone as well. Did you restore this iPhone from that iPhone’s backup? If so, I would recommend making another current backup of your iPhone, to ensure all of your data is safe, and then restoring your iPhone and setting it up as a new device to isolate the software as the potential issue. 

Information on creating a new backup and then restoring your iPhone and setting it up as a new device can be found in the links below. Please let us know the results of these tests so we can continue to help you troubleshoot this situation. 

How to back up your devices using iCloud or iTunes - Apple Support

How to erase your iOS device and then set it up as a new device or restore it from backups - Apple Support

Best!