I agree with Test-Ok, usually this points to the carrier. The easiest way to test it is to replace the T-1 card. Is the T-1 plugged into the switch directly from a smart jack or NI? Check the cables to the T-1 via any cross-connect or jack. Has this happened since day one of the T-1 install? Make sure the provider tests the T-1 from the smart jack or NI to the Central Office, that can save you some time, make sure the circuit is good to begin with before you start trouble shooting. Good Luck!