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I was the last weeks in Cuba and it took me while to understand why any call from Germany to my mobile, when I was offline, was charged two times: first the call to my mobile from Germany and then the call back to the mailbox in Germany and each time 0.99 and 1.39 Euro cent. Luckily I have had a list with me of the so called GSM-codes which one can disable such call transfers, for example with
#62#(to be keyed in into the dialer app) to deactivate any call transfer when not reachable. A complete list for the provider O2 is here:
https://www.qitec.de/blog/o2-gsm-codes-fuer-sie-recherchiert/
(the list is in German, but easy to understand).I don't know if other devices (Android or iPhone) support this through some kind of GUI or if we have something for this in the OpenStore. But, for us experts it's easy to use via the dialer app. You should have the list with you if you travel far away.