Q&A for How to Increase the Volume on iPhone

Use the side button(s) if your phone has them. Or you can swipe down from the top of the screen for notifications and settings and change it there.

Go to Settings > Sounds and Optics. From here you can adjust the ringer and alerts volume by using the slider. You can also choose whether you can adjust the volume from your buttons with the 'Change with buttons' option.

Try hard-resetting your iPhone, this may be a software issue that can be fixed with a simple restart. If not, take your iPhone to an Apple store or an authorised service provider to have it professionally inspected and fixed.

Unfortunately, you can't delete a selected group of contacts at once. You can only delete them individually, which can be a pain, so it may be worth looking into apps that can do this for you, however they're usually not free and you'll need to pay to download them.

Since it's likely a hardware issue, going to an Apple store is your best option.