As you know Teams is the meeting client replacing Skype for Business Online for online meetings.
Microsoft is currently offering 1 year free for call-in number.
The offer is available until March 31, 2021.
If you want to take advantage of this 1 year free of call-in, logon to your Office 365 administration portal (https://admin.microsoft.com/) and access the Billing\Purchase services blade
Then access the Add-ons category and search for Microsoft 365 Audio Conferencing Adoption Promo