Phones - Sangoma Phone Cannot Provision via DHCP Option 66

Issue:

The provisioning process times out when using DHCP Option 66 with Sangoma phones. Packet capture shows that SIP messages are being denied with a 401 Rejected message.
 

Solution:

Out-of-call messages for DPMA are not set to be handled by Asterisk in sip.conf. Add the lines below to allow the phones to provision:

accept_outofcall_messages = yes
outofcall_message_context = dpma_message_context
auth_message_requests = no