I second Dr. PBX. Very likely you're getting "smoke signals" from the provider. Is it a small "Mom & Pop?"
If it's not the provider, the most likely cause of unreliability in a PRI is that the it's set to the wrong ISDN protocol in CM AA06. This should be set to 20 for AT&T, 21 for Northern/DMS, and 28 for NI2.
Northern is sometimes called DMS, and ST&T is sometimes called National, but some providers will argue with you on it. Let them win, it's not important, just find out what they're set to, and match it.
If it's set incorrectly -- i.e. you've got AA06>xxxx>28 and they need AA06>xxxx>20, it may "sort of" work, because, as a certain mildly-famous NTAC tech is rumored to have once said, "The switch is just a bit slutty. Sometimes it'll go with whatever's there."
But as Dr. PBX so wisely points out, if you follow the cheat sheet, you'll almost always nail down the issue.
