Question about AVM FRITZ!Fon C5 International / CH

Has anyone had similar bad experiences with the FritzFon C5? The devices I received work for about 3 months and then can no longer be switched on. When pressing the red telephone key, the couvert key lights up followed by the red telephone key. However, the unit does not switch on. When I put it in the base station, the light is repeated, the unit tries to switch on, but switches off again immediately. I then have to remove the battery, wait a few days and recharge it, then the phone works again. What could be the reason for this? This is already the second device with this problem.