When joining a Teams meeting, you may not have access to a headset/microphone or conference room with teleconferencing abilities. Fortunately, Teams offers an option to use your phone to join the meeting without needing a dial-in number.
When you click to join the meeting, you will be presented with several options. At the bottom of this screen, you should see the option to use your phone, as shown below.
Click 'Phone audio' and enter the number you would like to use for joining the call
This will call you at the phone number you entered. Just answer the phone call and follow the prompts.
You can now join the meeting using Teams and your phone for audio.
The official Microsoft documentation can be found here.