Motorola A840

Has anyone experienced this intermittant but somewhat frequent problem? At some point, a call is made from the Motorola A840, it rings at its destination, but when answered, the call is dead at both ends. The ring is not heard on the originating phone. This problem persists, until the phone is reset by turning it off and on. Verizon has replaced the original phone after trying various network and programming fixes, but the problem persists. This has been experienced in GSM as well as CDMA mode. I'm at a loss as to what to do next, and would be interested as to whether anyone has any insight into this problem.