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 BillingPurchase services blade

Then access the Add-ons category and search for Microsoft 365 Audio Conferencing Adoption Promo