These files have a small typo in the way they check for permissions:
phonebox_plugin/templates/phonebox_plugin/voice_circuit_list_view_3.4.html
phonebox_plugin/templates/phonebox_plugin/voice_circuit_list_view_3.x.html
And they check for perms.phonebox_plugin.add_voice_circuit instead of perms.phonebox_plugin.add_voicecircuit.