Participants from Germany only can dial in - no dial-ins for US participants can be generated

Welcome to the Zoom Community, @GB8.

It's possible you only need to adjust which country's numbers are appearing.  Having changed from US-based to Germany-based Zoom Data Centers, take a look at this Zoom Support article:

https://support.zoom.us/hc/en-us/articles/115004454886-Specifying-Global-Dial-in-Countries-Regions 

The calling plan available by default is documented in this Zoom Support article, and appears to be independent of which Zoom Data Center is attached to your account:

https://zoom.us/zoomconference

The middle column is International (direct-dial) Numbers, and the right-side column is Tollfree Numbers.

Normally audio is easily obtained via the computer/internet, without resorting to telephone services. Before pursuing a costly telephone plan to add these, I'd suggest verifying whether or not they can participate fully via an internet connection only.