This depends on phone and version. Do you still have an iPhone with a physical ringer switch? Use that. The switch position towards you turns the ringer on. (I seem to recall that there was an ability to disconnect the switch with this setting in the System Preferences -> Sound options, but I now use an iPhone 15 so I cannot verify. But…)
Do you have a newer iPhone (iPhone 15 or 16) with the “Action Button” above the volume controls? Holding that button acts as your Ringer Switch by default, but it might be rebound to something else (System Preferences -> Action Button). If you have an Action Button, you should have the Ringer state in your Control Center. Swipe down from the very top right of the phone, and the Bell icon will either be “off” (like the majority of other buttons there) which is the normal state, where the normal state is when it WILL ring. But based off your description the Bell icon will have a slash through it. Tap it to cycle it.
One last possibility is that you have Notifications for the phone turned off. (System Preferences -> Notifications. Scroll down to Phone, tap it and review its settings.) However, these settings being turned off do not have are not consistent with the “bell with the slash through it” visual you described.
