It’s included with Office 365 E5 and Microsoft 365 E5 plans. For Business Standard you can get a free trial. After the trial, it’s available for $4 per user per month or as a pay-per-minute option.
https://www.microsoft.com/en-us/microsoft-teams/audio-conferencing?rtc=1
https://learn.microsoft.com/en-us/microsoftteams/audio-conferencing-in-office-365