SNOM 760 Does not indicate when DND is on when using feature code

[NSpinuzzi]

One of our customers has reported that turning on DND will not give an indication of whether Do-Not-Disturb is enabled, as it dials *72 and enables it on the PBX but does not communicate with the phone.
When the phone is manually configured via web interface to use the native DND rather than the feature code *72, it displays "DND" on the screen and the system does operate properly, going to voicemail when a call is directed to that extension.

The setup:
QX200, WAN port connected to local network and behind another firewall. The phones are configured on VLAN 2, on the Epygi's WAN port. The SNOM 760's are auto configured by the Cisco network infrastructure on VLAN 2, and all usual network communication works properly.