disconnection prb is usually as to do with line polarity, reverse current.
if ur signalling is in &&M mode & ur polarity is wrong, u may get disconnect after alart
try reverse the polarity of your PSTN line set ur signalling in E&M. better still.. try FXO signalling has that does not have much problem with polarity
also make sure u have all swithes on your Channel Bank if any; are set correctly as to your signalling