Hello,
If you are using Teams with a "personal" Microsoft account, PSTN call feature is not available. It requires business license plus some add on license.
For base license, you can use one of the following, make sure With Teams is selected.
https://www.microsoft.com/microsoft-teams/download-app#plansandpricing_tab1
if you do sign up for the business plan, it will have you create a new username that ends with onmicrosoft.com. Make sure you write that down because that's your user credential to log into Teams after that. And the subscription will be managed via this link https://admin.microsoft.com.
Once you are in the Admin Center, you can go to Billing > Purchase services, from there, you want to search for keywords "calling plan", and pick a plan that works for you. Calling plan info can be also found here: https://learn.microsoft.com/en-us/microsoftteams/calling-plans-for-office-365
For example, if you need to call internationally, these are two plans that could work. Price in USD. The only difference is, if you go with Pay as you go calling, you have to add communication credits yet, but the plan on the right includes 3000 minutes a month.
My honest opinion - It is not cheap. If you only need to play a few calls a month, you might be better off finding a third party option.