Incoming calls drop

Have been running asterisk with 3 port digim tm40b i think, for about a month
for some reason some incoming pot calls are dropped at random, some time after 1 sec , some times after 10 min, no pattern from what I can tell nor any errors in log, just looks like a normal hang up, i have searched the net for hours and don’t look like any one else has much problem with this or do they?