Which GSM / USSD codes have you tried?
Maybe: ##004# Alle bedingten Anrufumleitungen ausschalten (Besetztfall, nach 5. Klingeln, Nichterreichbarkeit)? Wikipedia is a good first stop, check “Weblinks” at the bottom of the specific page for more codes.
I’d presume the mailbox is a function provided by telekom (in your case) and not by phone-hardware or the OS or else. Therefore, I’d expect USSD-codes to work in general, least the basic ones to some extend…
Have you doublechecked with your provider?
(You might try USSD-codes whilst your SIM is in a “normal” phone in order to rule out USSD-codes not working because of eOS.)