iPhones and iPads have several independent volume settings. Some of these settings are available to all users. Others are available and relevant only to VoiceOver users. Here's how to adjust them.
The ringer and alerts volume is set in Settings / Sounds & Haptics. Below this setting is an option to allow or prohibit changing this volume with the volume buttons on the side of the phone. If you allow ringer volume to be changed by the volume buttons then the ringer volume will change if the volume buttons are pressed when the phone is unlocked and not playing some other media. I normally recommend turning this switch off since many people change their ringer volume unintentionally this way.
The volume of a phone call may be changed during a call with the volume buttons on the side of the phone.
The media volume setting controls the volume of music, audio books, Youtube videos, etc. as well as VoiceOver speech. This can be adjusted with the volume buttons on the side of the phone when you are listening to media or when VoiceOver is speaking. The media volume can also be set using Siri. A Siri command like "Set volume 70" or "Set media volume 70" will set the volume to the specified percentage of maximum volume. You can also use the Siri commands "volume up" or "volume down" to make small adjustments to the media volume.
The volume of Siri speech can usually be adjusted while Siri is speaking using the volume buttons. Ask Siri something that requires a long response such as "what's the weather" and adjust the volume with the volume buttons while Siri is speaking. You may also be able to adjust Siri's volume with a request like "set Siri volume 50", where 100 is maximum volume..
The volume of VoiceOver speech may be set as a percentage of the media volume. VoiceOver's percentage of the media volume may be set using the Volume setting on the rotor. This may need to be added to your rotor in VoiceOver settings. Be careful if you have Volume on your rotor. People have been known to use it and unintentionally set VoiceOver's volume to an inaudible level. I recommend that you don't have volume on the rotor unless you regularly need to adjust VoiceOver volume relative to media volume. This percentage can also be set in audio ducking settings described below.
If VoiceOver volume is set to less than 100% then some people may experience difficulty hearing VoiceOver during phone calls or Zoom meetings in order to access buttons. In phone calls and some conferencing systems audio ducking is disabled and the phone call may be too loud for you to hear VoiceOver.
VoiceOver has a feature called audio ducking which automatically lowers the media volume when VoiceOver is being used. The settings available for audio ducking are flexible and worth experimenting with. The settings are at Settings/ Accessibility / VoiceOver /audio / audio ducking. Audio ducking may be set to one of three values - off, when speaking or always. The "when speaking" setting will lower media volume only when VoiceOver is speaking and the "always" setting will lower media volume whenever VoiceOver is turned on. There is also a setting for VoiceOver speech volume as a percentage of media volume ranging from very low to 199%. You may find that a VoiceOver volume level higher than 100% makes it easier to hear VoiceOver while using the keypad during phone calls.
VoiceOver sounds are the pings and other non-speech sounds that VoiceOver makes. These can be turned on or off at Settings / Accessibility / VoiceOver / Audio / Sounds & Haptics. If sounds are turned on, you may choose to set their volume in one of two ways. If match speech volume is turned off, then the slider may be used to set the volume of sounds. If match speech volume is turned on, then sounds and VoiceOver speech will always be at the same volume level and if the Volume setting on the rotor is used it will adjust both VoiceOver speech and sounds volume.