Hello,
The F0 is always free. If you sign up for a Free (F0) tier instance, then once you hit your quota, you will no longer be able to proceed until that period is over.
Azure free account FAQ
https://azure.microsoft.com/en-gb/free/free-account-faq/
After speaking to a Azure Sales associate I was told "If you’re using the Azure AI Speech Free tier and you exceed the monthly free limit of 5 audio hours, the service will not automatically start charging you. Instead, the service will stop once the quota is exceeded. If you want to continue using the service beyond the free limits, you would need to upgrade to a paid tier", which would be the Standard S0 tier.
Just make sure you sign up for the Free F0 tier:
If this is helpful please accept answer.