Don't know what to do with control frame 15

I have this message appearing quite often on my Asterisk system (1.2.14), it doesn’t appear to be affecting the operation, but I honestly can’t tell. Does anyone know what is causing this? Our Asterisk system has 2 PRI channels and 5 VoIP peers (IAX2 and SIP).

Notice:
channel.c:2482 __ast_request_and_dial: Don’t know what to do with control frame 15