+49 89 721010 71711 is 15 digits long. That is the limit set by the ITU as the longest digit string required to be handled worldwide. However, just because the ITU says so doesn't mean that every carrier and every switch maker will be able to handle it immediately.
It is possible that your mobile phone service provider's switching network or their external gateway switch cannot yet handle such a long phone number. It is also possible that the number block (+49 89 721) is so new that they have not yet taught their switches how to reach it. This is a big problem in North America, where there are so many network changes each week that sometimes a change gets lost and must be manually entered. You should call your mobile service provider and explain that you can reach this number from your land line phone but not from their mobile phone and let their engineers resolve the problem.
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