Has anyone seen an issue recenlty in the last 90 days where ES400 on the ATT network will randomly shut down and cannot be powered back on without removing the batteries? Some of my customer's devices have been working fine in the field for nearly a year.  If a new SIM card is installed, 99% of the time the problem is resolved. Note that many of the ES400 have been using SIM cards from a previous generation of MC5574, so the SIM cards may be 3+ years old.  There does not seem to be any physical markings that indicate any difference in the SIM cards between new and old, which are 3G.  These SIM cards are suppied directly to the customer by AT&T, not thrugh an AT&T store.  The customer has severla hundred ES400 in use around the US. The problem accurs randomly around the US.