IAX2 Control subclass 255 (-1) question

Hi all,

I’m experiencing a strange problem with Voicepulse Connect whereas it stops all sounds upon accepting an outgoing call. The thing is that this is done by sending control messages with subclass 255 (-1) to my Asterisk which originates the call. I found no such control subclass code in the IAX2 specification (cornfed.com/iax.pdf). What is this subclass for?

app_dial.c stops sounds upon receiving the control frame with this subclass, unless forced ringing or MOH is specified in the Dial command originating the call.