how can i increase the volume on incoming calls on a iPhone 7 ?

See this for help, If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

Ringer volume is adjusted with the buttons on the side of the phone, as long as you have Change with Buttons turned on. That will raise and lower the ringer volume. As far as the number of rings before it goes to voicemail, that is not how it is measured. While it can be around 4 rings, it is setup by time. Your carrier has control over that, and some carriers can adjust it slightly, but not very long. It is usually around 30 seconds and that is counted from the time the call is setup and sent through the network to your phone. Good luck.

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