With a number of mobile phone providers in Germany, you may wish to change your network for various reasons. Switching to your preferred network is quite easy, even while keeping your current number. You only need to cancel your running plan and contact your preferred provider within 120 days before your contract expiry.
However, if your contract has ended, you can still contact another provider within 30 days of the termination date. You will get an exact date when your number will port to your requested network.