Quintum a800, lines always become busy

My a800 lines become busy a few time though the day, so i need to manually unplug and replug the PSTN lines to fix the problem,

the Lights arent on, and noone is being billed for any calls, but when you dial into the box youll get a busy signal. does anyone know about this problem and how to fix it? the only solution ive got is sitting next to the box all day unplugging lines as ports become free and then replugging again, but unforntunatly i cant always babysit the box, and this causes big problems for my calling card customers. please help