Based on this product's specifications, here are some alternatives and similar models
Replacement Battery FRITZ for Fritz!Fon X6 model 312BAT030, Li-ion cordless phone battery
Summary description:
Replacement Li‑ion battery for the Fritz!Fon X6 cordless phone (model 312BAT030) by FRITZ. Designed to restore original runtime and reliable power for calls, standby and handset functions. Ideal for replacing worn or degraded batteries in home or small office DECT handsets to ensure consistent battery performance and phone availability.