1. check the settings on espace 7820: "phone > features > General Information", make sure the configuration as blew:
we find in the abnormal phones, DND off code is blank. set it as #56# and re-test , the service is ok.
2. if the configuration on espace 7820 is ok, make a capture between IP phone and PBX, to confirm the phone have send #56# to PBX when press DND keys.
3. if the network capture is ok, then we need to trace a log on PBX (by ucmaint), to check why the PBX didn't disable DND service after receive #56#