Getting incoming calls from my own trunk?

For the past couple of weeks I’ve been getting incoming calls on my trunk that purport to be from my own trunk number. Each time I pick up the call it immediately gets dropped.

Nothing has changed on the system, there have been no changes to the config since the last time the system was setup over 10 months ago.

This is being used domestically in the UK and there is only one trunk line for my home phone number. The incoming trunk is handled by a Grandstream HT813.

Anyone have any insights as to what or why this is happening?

Thanks
-s