Just switched to simplytel.de. Great deals right now, as far as Germany goes. 5GB data, unlimited calls / sms, 1GB of roaming data in the EU, €20/month.
Plus, it includes the option to have 3 sim cards, what they call multicard. Been messing about figuring out how to switch call routing, etc.
*120# – cheat sheet (German)
*121# – show which sims are online
*122# – where are calls routed
*123# – route calls here
*124*P# – set this device priority P (1-3) or 0 to disable, doesn’t seem to work
*125# – route SMS here
*126# – route MMS here
These are all USSD codes which you run on the phones to configure the call routing. I couldn’t get the priority to work, it maybe isn’t supported by simplytel (Drillisch) as a reseller of o2, not sure. I got this info here.