Your Microsoft 365 users will require at least a "Microsoft Teams Essentials with AAD" or any Microsoft 365 plan + the "Microsoft Teams Phone Standard" add-on license to enable PSTN functionality (Receive inbound calls / Make outbound calls) with our Teams calling plan.
If you already have a Microsoft 365 E5/A5/G5 plan, then these plans will come included with the "Teams Phone Standard" add-on license.
As an authorised Microsoft CSP partner we can provide you with the required licenses if needed.
Please contact us if you have any further questions regarding your licensing requirements.