I have duplicated the issue on our in house CP20.
Yes, all cables have been removed from the SLT cards except for my butt set.
It happens if I call the extension directly from a desk phone or transfer from voice mail. Across both cards and daughterboards.
The analog extensions are in a dept group and calls are transferred to the master number but the problem happens when calling the pilot or extensions directly.

I've never ran across this before either but then again most devices I connect via analog ports are fax machines anymore and they usually pickup on the first ring and I don't typically pass caller id through to them.