...in this case you need to flash your phone.
Question: which phone you have? SL56 ?? This is the cordless phone (Gigaset). Or SL65 (maybe you don't write corectly the phone name).
If your phone is SL65, you need to flash with X65 flasher (or V-Klay), using DCA500 ; DCA510 or C55 serial cable (using testpoint).
For more details search on this forum.
Br,
dagos