T42G DND not working

I have 9 T42G devices using a hosted SIP solution provide by Buzznetworks (UK). The DND function just doesn't work i.e. calls still come through when DND is active. I have tried one of the T23s we have here and they work fine with the same settings.

The T42s have recently been flashed with the latest firmware and have had a factory reset within the last week.

Any ideas?