This is because of how Sipgate sends the incoming caller ID. As it is intended, it shows both who the call is for (in case you have multiple numbers configured on your phone) and who it's from, after a line break. Neither SNOM phones nor linphone, ekiga, etc handle this perfectly. You may either fix this only on your SNOM phone or on all connected clients:
Log into your sipgate.de account and select Einstellungen Show archive.org snapshot , go to Routing > Telefone > Voip-Telefon von ... and switch Optimierte Rufnummernanzeige für einzeilige Displays to An to disable showing your own phone number. Now it should both show the incoming caller's ID when the phone rings and log the correct number in your call logs.