Incoming call receives CANCEL when default gateway is down

I have a FreePBX setup as follows:

Phone Service (through SBC) to ETH0 (SBC is, FreePBX system is
Connection to my LAN to ETH1 (LAN default gateway is, FreePBX is

Yesterday my default gateway (router) died. Once this happened, calls coming into the FreePBX from my SBC on the network failed. I confirmed the FreePBX box was receiving the incoming call request via sngrep but as soon as it received the SIP message, it sent a CANCEL.

In addition, outgoing calls from desk phones delayed 15-20 seconds before ringing (so outgoing calls DO work but very slow to initiate).

I ended up configuring FreePBX with no default gateway (until the router gets replaced) and everything went back to normal (incoming and outgoing calls).

I could write a small cron script to enable/disable the gateway based on ping (ie ping it every few minutes and adjust) but was wondering if someone knew the underlying reason it causes this behavior?

You seem to be describing a case where Asterisk is the UAS. Only something acting as UAC can send CANCEL. Your description doesn’t make sense to me.

UAS: User Agent Server (receives INVITEs)
UAC: User Agent Client (sends INVITEs)

Ignoring that anomaly, my guess is that you don’t have valid reverse DNS information, either in the host file, or in an on-site DNS server.

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