After Congstar, as a Telekom subsidiary, had been limited in terms of LTE usage for a long time, the situation has now improved: current contract rates support both Voice over LTE (VoLTE) and Wifi calling.
After the LTE activation (apart from the legendary 7.2 MBit/s in the old 9-cent tariff) was still limited in the spring of 2018, there is now no longer any disadvantage apart from speed: if you have a suitable terminal device, you can use either 25 MBit/s or 50 MBit/s LTE including Wifi calling for an additional charge of 3 euros/month.
For me, the announcement came in the Congstar Forum just right: my Telekom-Magenta-M contract is expiring, and since the legislation on limiting mobile phone contracts to 12 months is not making any progress, I didn't want to commit myself for long. However, I am very satisfied with Telekom in terms of coverage alone, so that it has now (once again) become the low-cost subsidiary.
With the doubled download comes also a quintupled upload of 25 MBit/s instead of 5 MBit/s, when it might be helpful with the one or other Instagram post. After the consumption of the data volume, Congstar unfortunately (like all providers except some O² tariffs) throttles quite hard to 64 Kbit/s download and 16 KBit/s upload. Although 20 years ago you could use the internet with it, nowadays it might be enough for a push notification, but not for useful use.
This feature is particularly exciting if you don't have perfect LTE reception at home or in the office, but are still supplied with WLAN. In addition, you can be reached so cheaply abroad (even outside roaming) under your German number, provided you are logged into a local WLAN (which is actually easier to do in all countries than in Germany, which still has its roots in people's minds "Interference liability" thank you".)
What is still missing: the possibility of a multi-SIM with the same phone number (also available as eSIM, which Congstar supports in principle). If you want to use an Apple-Watch with LTE, you still need a contract directly with Telekom (or Telefonica or Vodafone). Based on how long it took Congstar to offer the LTE functions VoLTE and Wifi calling, I don't expect to have the corresponding support before 2022.
From 04.05. on I can enrich this article with a speed comparison between Congstar and original Telekom LTE - both the 50 MBit/s at Congstar and the 300 MBit/s at Telekom are maximum values that the network has to reach first.
In a speed test in the Telekom LTE network (which I have created as a "baseline" for further consideration), an iPhone 11 Pro achieves 233 MBit/s download and 12 MBit/s upload - the upload should also reach Congstar, the download should be throttled to 50 MBit/s.
For a good 30,- Euro per month with 12 GB data volume, Congstar certainly does not provide the cheapest offer in the Telekom network, but as far as I have seen, the best combination of speed (50 MBit/s), network coverage (Telekom LTE), features (VoLTE & WiFi calling) as well as a short contract period of one month. How much the missing support of the Apple Watch will bother me, I will notice in summer.
UPDATE 04.05.2020: Still no operator logo at Congstar on iPhone
Already several years ago there were the effects that a Congstar SIM on the iPhone no operator logo has represented. This has apparently not changed in the last 8 years (!), you still have to choose the "empty" provider (or let the mobile phone choose)
In practice, there are no disadvantages: the reception bars are displayed regularly, the provider is not displayed with the notch on current iPhone anyway. By the way: On Android devices like the Xiaomi Redmi Note 5 ("whyred") the provider is correctly displayed with "congstar".
The throttling to 50 MBit/s works as advertised: in the speed test you can see how the speed is first slightly higher and then limited. The upload is weaker than advertised at my location even with an original telecom contract, but with > 10 MBit/s it is completely sufficient.
Overall, Congstar delivers a round package for a maximum of 30 euros per month with just one month's notice with LTE and sufficient speed. The only thing missing is "Visual Voicemail" as an iPhone feature, otherwise the "equipment" is comparable to the original magenta rates of Telekom.
I'm going to stay with the cheap little brother of Telekom for now and see how the tariffs develop.