@yang liang Thanks for your response.
Purchasing the phone number using the free trial subscription or free credits is not supported, in-order to purchase phone number, you need to have Pay-as-you-go subscription which is clearly called out in this documentation.
As you are looking to acquire a phone number and you are using free trial subscription you can use the trail free phone number which is being provided by Azure communication service but in order to get this number your billing account has to be in USA.
For other geographies, please visit the Subscription eligibility documentation to determine where you can purchase a phone number.
Hope this helps, let me know if you still have further questions on this.