right now, in Germany you can only get "Microsoft 365 Business Voice (without Calling Plan)". For the Enterprise licenses like Office 365 E5, you can book Calling Plans as an Addon for a long time already.
Is it supported to combine "Microsoft 365 Business Voice (without Calling Plan)" and a Domestic Calling Plan, that I booked as an Addon e.g. for an E5 plan?
If you ask this question to Microsoft directly they would state this is not supported case as the Feature is not enabled for the same.
Technically speaking I have tested long back and the services should work without any issues. You can enable the licenses and use the service. The only problem that might occur in case you face any problems Microsoft will state it is an unsupported issue.
This can be used till Microsoft Officially Launches the Microsoft Calling Plan for Business Users in Germany.