Phone rings but no PJSIP log

Hi, I am setting up my first asterisk box, a VPS in the cloud, using Asterisk v.13.1 + FreePBX v.12.0. on Centos 6.6. I am only using the PJSIP stack.

I have an issue with an extension, hooked up to a GrandStream 1405 phone in the office, that rings every 30 minutes or so for 5 or 6 rings. :open_mouth:

The display tells me that an extension like 7070 or 500 is calling, but if I answer there is no sound.
I have no extensions with those numbers in my local configuration.
I have tried debugging from the CLI using

as suggested in
but nothing is being logged.

What am I missing?
Can the phone ring with no PJSIP traffic?
Is something else on the local network making my phone ring?
Should I run Wireshark on the local network?