how do i fix bt studio 3100?

i have 2 bt studio 3100 (ie) 2 handsets and 2 bases, one works but the other has gone funny, when someone rings me it does not ring, and when i look at the handset on its base the screen shows one black mark going to full black marks then repeats from begining on battery icon,also signal icon is flashing. the batteries are fully charged, and i have removed them and put them back.thankyou.