Hi,
Which MNO is your service based on?
Regarding your question on what DID people would prefer, I would appreciate if I could hand over calls to your service by SIP.
So you would provide me a SIP-URI like
1234567890@telnamobile.com and I could forward incoming calls to my German DIDs to you by myself, which would be very flexible and save costs.
Providing German (no matter if geographical or non-geographical) numbers would be quite a problem for you, as such may only be assigned to residents and you will be required to validate residency of each customer, which means high administrative effort besides the costs for the rental of DIDs. Afaik your parent company 3U Telecom owns only number blocks in few cities.
regards,
Inquisitor