It is important to know how to turn off speakerphone during a call without hanging up on the caller on the other end. Tap the round, highlighted Speaker button on your iPhone screen. This button has a picture of a speaker and says "Speaker" beneath it. By turning this off you will reduce the amplification of the sound from your iPhone speakers and return to normal phone mode. If you find that your iPhone always answers on speakerphone, you may want to use the following steps to turn off the default speakerphone option. Accessibility options allow you to customize your phone to meet your special needs and preferences based on vision and hearing, or based on the environment where you commonly use you iPhone.  Unlock your iPhone and tap on the Settings icon. Scroll down and tap on the General option. Scroll down and tap on the Accessibility option. Apple has provided options for your call always being answered on a headset, speakerphone or automatic. You might choose one of these options if you live somewhere that requires hands free devices while driving.  Scroll down and tap on the Call Audio Routing option. Select Automatic from the menu options, you will see a “check mark” next to the select option”.

TL;DR in Vietnamese:
Tắt loa ngoài khi đang nghe điện thoại. Mở các tùy chọn Accessibility (Trợ năng) của iPhone. Tắt loa mặc định.