На моем iPhone 11 отсутствует рингтон

Are any other Bluetooth devices connected to your iPhone, for example headphones?

Just to be sure, did you check your Ring/Silent Switch on the side of the iPhone? Does it work?

Any options like "Do not disturb" enabled?

Turn on or schedule a Focus on iPhone - Apple Support

Включение или планирование включения фокусирования на iPhone - Служба поддержки Apple (RU)

Are you using 4G network? Is your cellular signal strong enough?

Are you using the standard ringtone that came with iOS?

Did you already try to force restart your iPhone?

Force restart iPhone - Apple Support

Does this happen only if you make a call, or does your iPhone only ring once, if you receive a call?

Are you using the Wi-Fi Assist for phone calls?

If yes, try to disable this feature.

About Wi-Fi Assist - Apple Support

Сведения о функции «Помощь Wi-Fi» - Служба поддержки Apple (RU)