Your orders can get you out early. German law has a route for exactly this: you're moving abroad, the provider can't follow you, so one month's notice and you're done, counting from the day you leave.
Section 60 (2) of the German Telecommunications Act covers a permanent move abroad where the provider can't deliver at your new address. One month's notice, effective from your move-out date, not from the end of some minimum term you're stuck in.
Telekom names military transfer orders as accepted proof, in writing. Other providers usually take them too. A few make it harder than it needs to be, and we'll tell you which before you start.
Only one page is ever attached: the one showing your report-no-later-than or detachment date.
Names of dependants, unit movement details, identification numbers and anything after a distribution marker are blacked out on your own device. The redaction is burned into the image rather than laid over it, and afterwards we read the page back to check the hidden text really has gone. If any of it survives we refuse the file rather than send it.
This part is worth checking before you start. If there's no minimum term left on your contract, ordinary notice already ends it within a month, and the special route ends it on your departure date instead, which may well be later. In that case your orders buy you nothing, and we'll say so rather than have you scanning and redacting for no reason.
Orders are worth using when a minimum term is in the way. In that situation they can save you several hundred euros.
No. U.S. forces members are exempt from German registration under the Status of Forces Agreement, so there's usually nothing to deregister from. Providers ask for the certificate because that's what a German civilian would hand over.
Yes. What matters is that the provider can't deliver at your new address. Another OCONUS posting counts just as well as a move home.