The Grandstream Series DP7xx have two methods to for Do Not Disturb (DND) functionality. You can either toggle DND on a per account basis or for the handset itself which simulates DND for all accounts on the handset.
If you find that you are not receiving incoming calls and notice a DND symbol on the phones screen, follow the instructions below to turn off DND.
Toggle DND per account (if you have multiple accounts registered to the phone) can be configured under Menu > Call Features > Do Not Disturb > Account:
The result of enabling DND here is that account 1004 is on DND only which will result in calls being rejected to account 1004. The below image shows the DND icon next to account 1004:
To toggle DND for all accounts on the phone simply press the mic button (Mute as DND) which will apply DND to the handset and display the icon in the top left of the screen (all accounts on the handset will reject calls):
You can also disable the Mute as DND option if this is desirable under Preferences > Mute as DND > Disabled > Save: