Since I'm no sure if Fairphone are really aware of the problem, because it only affects users of some providers, I started investigating a bit about the problem.
In this article: http://alltech-reviews.de/oneplus-two-killt-telekom-sim-karten/ I found a statement of Telekom explaining the problem:
I already contacted Telekom and they told me the following: Their SIM cards use a feature called „MEC-Request“ (Mobile Equipment Change). Those requests contain things like activation, MMS-configuration, RDM-settings and more. The OnePlus cannot deal with these features and gives wrong feedback to the SIM card, so the SIM card tries it again and again, every 30 seconds, until the SIM cards memory flash cells break and the SIM becomes faulty. Here’s the German answer from Telekom: https://telekomhilft.telekom.de/t5/Handys-Datenger%C3%A4te/AW-Defekte-Sim-Karte-beim-Oneplus-One/td-p/1589987/page/2
* Deutsche Telekom
* Klarmobile and Callmobile (only older SIMs, now they use the Vodafone network)
* T-Mobile Austria
Affected SIM card types:
* C/W2 Hybrid
* C/W3 nano
SIM cards that seem NOT to be affected:
* V2 (Congstar, old)
* B/V2 (Congstar)
* C/V7 (Congstar, Deutsche Telekom)
* C/T6 (Deutsche Telekom)
* T-Mobile Austria say they have a compatible one, but I can't find the version number
Other phones that are affected:
* OnePlus One, One Plus Two, One Plus X
* LG G2
* Oppo Find 7a and N1
* Xiaomi (Mi2S, Mi3 and Mi4)
* others reported in the Congstar forum: Nokia Lumia 630 Dual-Sim, ZTE KIS PLUS, Lenovo Yoga 10 hd+ Tablet, Samsung Galaxy -GT18150, ZTE OPEN C, Yotaphone 2, InFocus, LG E-400, Phicomm Energy M E550
I also found the info that there is an update for the Oneplus Two that contains 'Ultra SIM compatibility improvement', so maybe there is a possibilty that Fairphone can fix the problem with a software update.
Meanwhile, I would recommend the clients of the affected providers to insist on getting a compatible replacement SIM (C/V7 or better C/T6).
When Congstar started to send out the C/V7, the complaints in the forum stopped, so there's hope. I got mine on 9th March and had no problem so far.