Unknown IE 128 (cs0)

I’m seeing the following in my Asterisk console:

This is followed by a hangup, with the cause being any of the following: 31, 34, 54, 102, 122. That’s just the ones I noticed looking back through the log.

Any idea on this one?

Asterisk 1.8.6
Digium Wildcard TE110P T1/E1 Card
libpri 1.4.12