Hi Iniobong
Do the users placing this calls have 'Communication Credits' as a license assigned to them?
Otherwhise this is impossible.
If the license 'Communication credits' is assigned to the user, you can check in the Azure Portal (https://portal.azure.com) if the balance is 0.
When it is 0, you will also need to buy additional credits for this.
Can you check and let me know if this resolved your issue?
Kind regards
Sam