I can confirm that I can’t disable those notifications for the SIM Toolkit App from Telekom on my phone as well (I never got such notifications, though) … but it seems I could make them silent if I wanted, see Settings - Apps & notifications - Show all … Apps - SIM Toolkit - Notifications:
The notification switches here can’t be switched, but “Mobilfunkdienst-Nachrichten” can be tapped here, which for me leads to …
… Once again, the overall switch does nothing, but silencing (“Lautlos”) can be set, as well as some of the advanced settings (“Erweitert”) …
… including disabling vibration and setting the notification sound to none.
Fairphone 3 on 1.3-q-20220825213388-dev-FP3,
Settings - About phone - Android version (tap on it) - /e/OS version (can be long-tapped and copied to clipboard since Android 10)