Is it safe to buy-and-forget prepaid SIM card with internet tariff in Germany?

I bought a congstar prepaid Internet stick in December 2012 in a
Kaufland supermarket in Berlin. Registration was via the congstar
web site, and done in some minutes. There are several thirty day data
packages available, most notably: 1GB at 12.90 EUR and 3GB at 19.90 EUR.
congstar is a discount daughter company of Deutsche Telekom (T-Mobile), and
network coverage of Deutsche Telekom is perhaps the best in Germany.

Concerning your question: If you subscribe to a data package, it will
only be extended if there is sufficient credit on your
card. There is no deactivation fee. Quote from the section for the
1GB data package, Surf Flat Option 1000, in the current price
list (German):

Die Buchung und die Wirksamkeit der Option setzt voraus, dass der
Kunde zum jeweiligen Abbuchungszeitpunkt des monatlichen
Optionspreises über ein ausreichendes Guthaben auf seinem
Guthabenkonto verfügt. Der Abrechnungszeitraum der Option beträgt 30
Tage. Die Option verlängert sich automatisch jeweils um 30 Tage, wenn
sie nicht mit einer Frist von einer Woche zum Ende eines
30-Tage-Zyklus gekündigt wird.

Weist das Guthabenkonto zum Abbuchungszeitpunkt kein ausreichendes
Guthaben auf und kann der Optionspreis nicht abgebucht werden, gelten
die Konditionen der Option bis zu einer erfolgreichen Abbuchung nicht
mehr und die Datennutzung wird nutzungsabhängig nach dieser Preisliste
abgerechnet. Soweit die Option nicht gekündigt wurde, führt congstar
maximal bis zu einer Dauer von 180 Tagen regelmäßig einen
Abbuchungsversuch durch. Wenn dieser erfolgreich ist, erhält der Kunde
eine SMS-Information und die genannten Verbindungen werden wieder zu
den Konditionen der Option abgerechnet. Andernfalls wird die Option
gelöscht.

As Google Translate struggles with this text, a rough translation done by me (disclaimer: IANAL – I am not a lawyer):

The booking and validity of the option (i.e. data package) requires that, at times of billing of the monthly option price, the customer has sufficient credit on his prepaid account. Billing period for the package is 30 days. The option automatically extends for 30 days, unless it was cancelled at least one week before the end of the 30 day cycle.

If, at the time of billing, the prepaid account does not have sufficient credit and the option price cannot be withdrawn, then the option will not be valid anymore and data usage will be billed according to usage as described in this price list. If the option hasn’t been cancelled, then congstar will repeatedly attempt to withdraw from your prepaid account up to a maximum of 180 days. Once an attempt has been successful, the customer gets an SMS information and data connections will again be billed according to the option. Otherwise, the option will be deleted.

Just to clarify: Guthabenkonto refers to the virtual account where the money is stored that you charged your card with. Above, I translated it as prepaid account. Guthabenkonto does not refer to your bank account, which you don’t have to specify, unless you want automatic recharging for convenience.

Before 2017-07-01

Sellers are required by law to record your name+address+birthday when you buy a SIM card. But they are not required to verify that it’s your real data.

When you have to register the card in the shop (probably the case for T-Mobile), you’d typically have to show your ID. But when online activation is required, you don’t have to enter your real data.

So, look for such a SIM card with online activation. You typically find them in discounters (near the tillpoint), fuelling stations, or at newsdealers (often at train stations). The dm drugstores (also often at bigger train stations, but in almost every city, too) also sell prepaid SIM cards (Fonic).

See also in Wikipedia: Liste der Mobilfunk-Discounter in Deutschland

New rules starting on 2017-07-01

Sellers will be required to verify the buyer’s identity by checking suitable identity documents (e.g., ID cards or passports).

I’ve never heard of any pre-paid provider charging a “deactivation” fee, and I can’t find any reference to T-Mobile having such as charge.

However even if they do, no carrier is going to come after you over a single-digit euro charge. You can safely just throw away the SIM (or keep it until your next trip to Germany – T-mobile SIMs are good for at least 12 months) with no risk.

The requirement to collect identification when activating a SIM is something set by the German government, and not something T-mobile does specifically. In my experience, they do not care if the information you give is correct or not – as far as my German SIM provider is concerned I live at a coffee shop across the street from a hotel I once stayed at!

I don’t know about T-Mobile, but I can tell you about Aldi-talk, which have very good prepaid data flatrates, and I guess that most of the prepaid have the same rules.

• When you buy data flatrate for a month, you can deactivate it without any fees, and when you need it you can activate it again.
• If you don’t have enough money for the flatrate in your card, your flatrate will not be activated automatically for the next month

So it’s not problem at all to buy a card, to use it for a month or two and then to throw it away. You can activate an Aldi-Talk SIM-card online, where you have to give an adress in Germany. But nobody controls that and you don’t have to give your ID-Card.