I’m pretty sure if I accidentally press “OK”, I get charged extra. In addition to the pop-up, I get the same thing as a permanent notification until I reboot – if I try to disable the notification, I get “These notifications can’t be modified”, as it is using the built-in system SIM Toolkit. I am using GrapheneOS. Name and shame: MOVISTAR.
Have you tried dev mode settings? I definitely remember having more granular permissions over notification controls after toggling something in there,though I forget what.
The grapheneos forums may be able to help you as well.