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:

SNOM phone fix only

• Log into the web interface of your phone (yes, your phone has a web server) by opening its IP address in a browser. You can find the IP address in the phone's Menu under "Information" (~11th menu item).
• Go to "Setup / Preferences"
• Change the "Number display style" to "Number"
• Scroll down the huge list
• Press "Apply"
• Scroll up the huge list
• Press "Save"

All clients fix

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.