Hi,
we had a simalar situation with Avaya 9620 phones.
Two phones were connected to the network and working properly, but by accident connected the LAN ports on both phones with each other, creating a loop.
Because "spanning-tree bpdu-protection" was configured on both switchports, both ports were put in blocking state.
So my answer is to configure bpdu-protection on edge ports.
Regards,
Michael