PSTN calls dropping intermittendly

Please can someone shed some light on why calls incoming and out going on PSTN lines are disconnecting intermittendly there is no exact time limit.

I have looked for config files that have info on time limit etc but cant find anything.

what kind of interface card are you using? run zttool adn see if you are getting missed interupts

I am using a digium TDM 400P Card with 4 analog ports (2 used) my phones are act 106 and sipura spa 841. I see there are no interupts missing.
Can you think of thing else