For me, although the Alarms-Only do-not-disturb mode works as expected, Priority-Only does not-- this supposed to let through only calls in my contacts list (or, optionally, only the "starred" contacts), and does so for native cellular calls, but all incoming Zoiper calls ring through when DnD is set to Priority, regardless of the calling number.
This is under Android 7.0 on a Motorola Moto G5+ XT1687, running Zoiper 2.7.12, Library revision 2.9.2. In case it matters, I'm connecting via IAX protocol to an Asterisk server under my control.