The software versions of the base station and the handset are not compatible. A compatibility update is necessary. For this update Gigaset Quick Sync Software is required:
Windows: Version 8.2.xxx or later
Macintosh: Version 3.02 or later

Download: https://www.gigaset.com/hq_en/cms/service/downloads.html
With this guide, you will be guided step by step through the update. 
 >Compatibility Update Gigaset SL910/SL910A/SL910H 
If there is no possibility to process the update, please contact the Service hotline. In this case the base station and handset must be repaired in the repair center.