Outbound calls. couldn't complete the call, check you are using the correct number format

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By documentation an integration needs to be done first with Telstra PSTN service for you to make outbound\inbound PSTN call from Microsoft 365 (S4B\Teams):

• https://docs.microsoft.com/en-us/microsoftteams/country-and-region-availability-for-audio-conferencing-and-calling-plans/availability-in-australia
• https://www.telstra.com.au/business-enterprise/products/unified-communications/collaboration/calling-and-productivity/telstra-calling-for-office-365

 Not sure if this is completed prior to your testing, but I'd suggest to contact first Telstra for integration process as you've already provisioned services\user on your end.

Detailed Instruction:

 https://docs.microsoft.com/en-us/microsoftteams/manage-phone-numbers-for-your-organization/phone-number-management-for-australia?toc=/skypeforbusiness/toc.json&bc=/skypeforbusiness/breadcrumb/toc.json

https://www.telstra.com.au/content/dam/tcom/business-enterprise/collaboration-conferencing/pdf/telstra-calling-for-office-365-admin-centre-user-guide.pdf

Polite as usual,

BFN

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