ALL NET FLAT OPTION
(FLAT means unlimited) This option has unlimited free calling and sms and several internet (LTE) options starting from 2GB to 8GB. You can either have a contract for 24 months which starts from 9.99 euros ( always check the final cost, sometimes it might be higher than the costs shown). But, you can terminate the contract only with a 3 month notice period. The same option is also available as monthly plan in which you terminate the contract with just 14 days notice period.
Such contract based options are better if you are staying in Germany for a long time as a student or working professional.
SMARTPHONE FLAT OPTION
This option is cheaper compared to ALL NET FLAT but, with restricted minutes, costs for SMS and less internet options. Again, you can buy this with a contract for 24 months, or choose the monthly option which can be terminated with 14 days notice period.