Some customers prefer not to delegate access rights to the Call2Teams portal to do this for them, even though it only uses a temporary token that expires after each sync, and wish to undertake the setup process manually.
This manual setup process should be undertaken by a competent Office 365 administrator who has good PowerShell experience. If there is already some PSTN integration set up on the tenant, then these instructions will need to be modified by an Office 365 PSTN voice expert to ensure they do not impact the existing configuration.
The configuration is split into two sections: Tenant Configuration, which is performed once on initial setup and again whenever a new region is required, and User Configuration, which is performed for each user as they need to be enabled for voice.
Please see Section 5 of the Teams Admin Setup Guide in the Documents section of your Teams Service screen: