When you are leaving, you should close your electricity and telephone accounts. For the telephone account, you have to find out how far in advance you should give notice and then send in a written notice in due time.
To close your electricity account, you have to go to your electricity provider (e.g. SaarLorLux) and close your account. You then have to pay any outstanding fees. If your accommodation is provided by the MPI-INF then you don't have to go to the electricity provider yourself. This is normally done automatically and if there are any electricity fees outstanding, it is deducted from your deposit.