All-Network Flat-Rate Tariffs
Internet use by Smartphone
All tariffs offer Telephone and SMS flat rate to all German networks. Few tariffs offer a real Data flat rate.
Data Volume 6 to 120 GB (SIM-only Tariffs)
No real data flat rate below EUR 50 per month
Telekom MagentaMobil S
39.95 EUR/month: 6 GB data volume with LTE Max up to 300 MBit/s download, up to 50 MBit/s upload. Afterwards throttling to 64 KBit/s for download, 16 KBit/s for upload. WLAN hotspot flat rate. EU roaming incl. Switzerland. Max. 10 additional SIM cards with the same phone number (MultiSIM) for use e.g. in a tablet PC – 4.95 EUR/month each.
Savings tip for these two Telekom tariffs: if no WLAN is available and the data volume of the tariff is to be spared or used up, I recommend booking a DayFlat unlimited (24 h unlimited high-speed volume for 5.95 EUR).
Telekom MagentaMobil M
49.95 EUR/month: 12 GB data volume with LTE Max up to 300 MBit/s download, up to 50 MBit/s upload. Afterwards throttling to 64 KBit/s for download, 16 KBit/s for upload. WLAN hotspot flat rate. EU roaming incl. Switzerland. Max. 10 additional SIM cards with the same phone number (MultiSIM) for use e.g. in a tablet PC – 4.95 EUR/month each.
Vodafone Red M
49.99 EUR/month: 30 GB data volume with LTE up to 500 MBit/s download, 100 MBit/s upload. Afterwards throttling to 32 kbit/s - quasi offline and monthly forced activation of 3✖250 MB â 3.00 EUR, if you do not opt out via SMS reply – monthly!
Conference calls (calls with more than two participants simultaneously) are not covered by the flat rate!
Telefónica O2 Free L
39.99 EUR/month: 60 GB Data volume with LTE Max up to 300 MBit/s download, up to 50 MBit/s upload, then throttling down to 32 KBit/s – quasi offline! Ready for 5G. Internet telephony (VoIP). Landline number included free of charge for nationwide reachability.
Telefónica O2 Free L Boost
44.99 EUR/month: 120 GB Data volume with LTE Max up to 300 MBit/s download, up to 50 MBit/s upload, then throttling down to 32 KBit/s – quasi offline! Ready for 5G. Connect option (use data volume on several mobile devices): You get 2 SIM cards (multicards) at no extra cost, on request as eSIM (for the same telephone number) for using the telephone and SMS flat rate as well as the Internet. Book up to 8 SIM cards for other mobile devices and manage SIM profiles at any time and free of charge via «My o2» app. All SIM cards may use the data volume included in the tariff. Internet telephony (VoIP). Landline number included free of charge for nationwide availability.
Real Flat Rate without Device (SIM-only Tariffs)
Unlimited data volume from 50 EUR/month
Telefónica O2 Free Unlimited Max
49.99 EUR/month: unlimited high-speed volume with LTE Max up to 300 MBit/s download, up to 50 MBit/s upload. Ready for 5G. Internet telephony (VoIP). Landline number included free of charge for nationwide reachability.
Telekom MagentaMobil XL
84.95 EUR/month: unlimited high-speed volume with LTE Max up to 300 MBit/s download, up to 50 MBit/s upload. Tariff enables 5G use, if already receivable on site.
Max. 2 additional SIM cards with the same phone number (MultiSIM) e.g. for your tablet PC per MultiSIM a whopping 29.95 EUR/month!
Real Flat Rate with Device (Mobile Phone Deals)
Unlimited data volume from 80 EUR/month
Telekom MagentaMobil XL Premium
(read my German-language test review)
199.95 EUR/month: Unlimited high-speed volume with LTE Max up to 300 MBit/s download, up to 50 MBit/s upload. Annually strongly discounted top smartphone with contract extension for another 24 months. Telephone and SMS flat rate to all German networks. EU-Flat Plus (EU, Switzerland, Norway, Iceland, Turkey, USA and Canada). Free landline number if required (only by calling customer service!). Max. 2 additional SIM cards with the same number (MultiSIM) e.g. for your tablet PC free of charge. Additional MultiSIMs per piece a whopping 29.95 EUR/month!
Vodafone Red XL Unlimited
79.99 EUR/month: Unlimited high-speed volume with LTE up to 500 MBit/s download, 50 MBit/s upload. Telephone, SMS and MMS flat rate to all German networks. With annual contract extension for a further 24 months, strongly discounted top smartphone. Max. 2 additional SIM cards with the same phone number (MultiSIM) for use e.g. in a tablet PC, each 10.00 EUR/month! Per MultiSIM 10 GB volume/month – no unlimited flat rate!
… or do you need more data volume only stationary or on the tablet PC? Then take a look at these data tariffs …
Although the above-mentioned smartphone tariffs are more expensive than those of resellers (providers such as mobilcom-debitel, Drillisch etc.), I recommend concluding a contract directly with the network operator (i.e. only Telekom, Vodafone or Telefónica). To put it diplomatically: it is less stressful. 
Recommended distribution channel: retail shop. There can be very attractive promotions online for a limited time. For working people, however, these are usually not usable if SIM cards are only handed over personally for security reasons (Deutsche Telekom, maybe also Vodafone). Either you are not met by the delivery person or the delivery person does not come to your door at all. Online offers can usually also be used as an «offline regular customer» and you can book yourself in the customer centre (web portal) of the network operator. It is often better to first contact the mobile phone retailer of your choice locally/via the Internet. They may be able to make you a better offer!
It doesn't cost anything to ask, but it can be worth it!
Watch out! All work on a commission basis and therefore not everyone gives priority to customer requests!
If you do not trust your local dealer, you should buy online and get further opinions first. Some online forums are very suitable for this!