What to do when your iPhone 7 has low call volume?

If you are talking about your Ringtone being low on volume, you can go into Settings > Sounds & Haptics and select 'change with buttons'.

If you are talking about the volume is low when you are listening in a call, while you are in a phone call, press the volume up key.

You can also check to see if there is any debris or dirt on the earpiece grill and clean that off or check if anything is blocking the grill (for example your screen protector). Hope this helps!