Note:
There are a few conditions that you need to comply with in order to enter a valid Test number and start testing.
    1. The test number cannot be empty. It should at least have one entry.
    2. A test number must have a minimum of four digits and should not exceed a maximum of 20 digits.
    3. A test number must start with a plus sign '+' followed by the country code and then the number. (Ex: +1 331 204 0199)
    4. You can associate the test number with any of the available channels and start testing.

The final step is to send a test message from the Sandbox environment and to respond back from your test number. In this step, you will be responding to a user-initiated conversation through WhatsApp.

So easy, right!
Let's start a conversation by sending a message from your test number.
Happy Testing!

Select the Sandbox tab on the WhatsApp Integrations page. Then, click on the horizontal ellipsis at the right of your test number entry, followed by Update Test Number. Make the necessary modifications and click Save on the Update Test Number screen.
Tip: You can update one or all the sandbox numbers in one shot.

You can also customize your preferences settings to specify things like who can respond to your messages and when to convert conversations into tickets in the Sandbox environment. Click here for more details on preferences settings.