Now we've come to the end of the contract and, having been given new phones we're looking to offload the Desires, but that when the problem manifested itself.
One potential customer, who's on the ASDA network asked me to try their SIM in the phone. This I duly did, but all that happened was that the lock screen appeared and then froze. All I was able to do was scroll down the "Settings" screen, but attempting to access any of the categories just dumped me back on the lock screen.
This means that I'm unable to unlock the phone as, even if the phone recognised a SIM I have no way of inputting a PIN. I then obtained SIMs from several different carriers, with identical results. I even tried restoring one of the handsets to its' previous, un-rooted state but this made no difference. Up until the freeze the boot sequence seems normal.
If it helps, when this happens the taskbar shows a small SIM-like icon with a cross next to it and "Emergency calls only" along with a completely greyed-out signal strength indicator so I'm at a real loss as to where to go from here- any suggestions? Thanks for looking.