call not silenced under sleep mode while hooked to speaker

If you have another Apple device (like a Mac, iPad, or Watch) nearby that isn’t in Sleep Focus, that device could’ve disabled Focus if share across devices is enabled.

When your Mac is set to "Do Not Disturb" (DND), your iPhone will also be automatically switched to "Do Not Disturb," and vice versa. To prevent this synchronization, consider disabling the "Share Across Devices" option, as illustrated in the image below.

Please look at your settings...

Repeated Calls: If someone calls you twice within three minutes, the second call isn't silenced.

Please see the pic below which is self-explanatory

• You have allowed certain notifications to bypass Do Not Disturb. By default, phone calls, text messages, and FaceTime calls will still ring or vibrate when Do Not Disturb is on. However, you can also allow other notifications to bypass Do Not Disturb by going to Settings > Focus> Do Not Disturb > Allow Notifications and selecting the apps and contacts that you want to be able to notify you even when Do Not Disturb is turned on.
• You have enabled System Haptics. System Haptics provides haptic feedback for certain actions, such as when you touch and hold an app icon or scroll through a list. Even if Do Not Disturb is on, your iPhone will still vibrate for System Haptics if you have enabled it. To disable System Haptics, go to Settings > Sounds & Haptics and turn off System Haptics.

What Call? On an iPhone, you can make and receive various types of calls. Here are the different types:

1. These are standard voice calls made over the cellular network. They utilize your carrier's voice network to connect you with other phone numbers. Allow phone calls on your iPad, iPod touch, and Mac
2. FaceTime Audio allows you to make voice calls over Wi-Fi or cellular data using your internet connection. It provides higher audio quality compared to traditional cellular calls.
3. FaceTime Video enables you to have video calls with other FaceTime users. It uses the front or rear camera on your iPhone to transmit live video during the call. Use FaceTime with your iPhone or iPad - Apple Support (IN)
4. VoIP (Voice over Internet Protocol) calls use internet connectivity to make phone calls. Apps like Skype, WhatsApp, Viber, and Google Hangouts utilize this technology, allowing you to make voice and video calls over the Internet.
5. Some carriers support Wi-Fi calling, which allows you to make and receive calls over a Wi-Fi network instead of relying solely on cellular coverage. This can be useful in areas with weak cellular reception. Make a call with Wi-Fi Calling - Apple Support (IN)
6. The App Store offers numerous calling apps that utilize different technologies. These apps can provide additional features and options for making calls, such as international calling, call recording, and more.

It's important to note that the availability of certain call types may depend on your iPhone model, iOS version, and carrier.