Sipura ATA 100 fantom rings phone intermitantly

All along that I have had my asterisk system running always thought it was asterisk that would sent a 1/2 second ring signal to the ata. The ring would happen mabey once a day or twice a day some times very late in the evening. The one time I had the ata disconected from the cat5 cable it rang the phone again. I said well, we have a ata buggy issue. Do these units do this because of a slight misconfiguration? Or is there a known bug on these older units that exibit this strange fantom rings?