Going unreachable occurs when chan_sip sends a SIP OPTIONS and gets no response.
Explanation:
If MOST or ALL your Asterisk serveres are behind NAT, seeing them go offline is not uncommon. This usually happens when the firewall has seen no traffic on the UDP port for a certain time, and decides to drop the entry from the NAT table.
Possible solutions:
In no particular order.
Decrease the registration time if the devices register with each other. This will keep the NAT entry active.
Increase the UDP NAT timeout in the device performing NAT on the networks.
Change from SIP over UDP, to SIP over TCP, most NAT devices will keep TCP connections open until they see a FIN or RST packet.
https://community.asterisk.org/t/aug-17 ... 36/98260/5
