Call audio automatically switches to speaker then disconnect automatically.

There could be a few reasons why a voice call gets disconnected when you put an iPhone on speaker mode. Here are a few possible explanations:

1. Poor network coverage or a weak cellular signal can lead to call drops, especially when using speaker mode. The speakerphone feature may require a stronger signal to maintain a stable call, and if the signal strength drops significantly, the call may be disconnected.
2. When you switch to speaker mode, the iPhone's microphone and speaker configurations change. If there is interference or a hardware issue with either the microphone or the speaker, it can result in call disconnections. Wireless interference: Power lines, radio, or television. transmitters, transformers, and other electronic devices may cause noise on the phone line. --> Resolve Wi-Fi and Bluetooth issues caused by wireless ...
3. iPhones typically have a proximity sensor that detects the position of the device relative to your face during a call. This sensor turns off the display and deactivates the touch screen to prevent accidental inputs. If there is a problem with the proximity sensor, it may mistakenly detect that the phone is close to your face, even when in speaker mode, leading to call disconnections.
4. It's possible that you are accidentally pressing a button or gesture that turns on/off the speaker. Try being more mindful of your movements during a call to see if this is the issue.

If you're experiencing this problem of disconnection while on a voice call, it is recomended to try the following troubleshooting steps:

1. Ensure you have a strong cellular signal and good network coverage.
2. Restart your iPhone and try again.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
3. Update your iPhone's software to the latest iOS version.
4. Disable any Bluetooth connections, as they can interfere with speakerphone functionality.
5. Reset your iPhone's settings to default by going to Settings > General > Transfer or reset iPhone > Reset > Reset All Settings. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
   1. Reset All Settings: Return iPhone settings to their defaults - Apple Support:- Return iPhone settings to their defaults
   2. You can return settings to their defaults without erasing your content.
   3. If you want to save your settings, back up your iPhone before returning them to their defaults. For example, if you’re trying to solve a problem but returning settings to their defaults doesn’t help, you might want to restore your previous settings from a backup.

If the issue persists, consider contacting Apple Support or visiting an authorized service center for further assistance.