Channel.c: Unexpected control subclass '16' (Ast

I am using Asterisk, no special channel drivers as we’re pure SIP/RTP with extension to extension calls between SIP clients in my own network (no cards or outside PSTN as this is a closed system).

We are using queues where a group of the extensions call in to a queue (hotline style) and queue up waiting for extensions who log in to the queues as members to answer the queued calls. Pretty simple call center setup, very minimal.

While for the most part we have completely normal operation, occasionally we’ll get this error in the console and messages log file saying this:

[quote][Mar 27 07:28:05] WARNING[28195] channel.c: Unexpected control subclass ‘16’
[Mar 27 07:28:54] WARNING[28196] channel.c: Unexpected control subclass ‘16’
[Mar 27 07:28:54] NOTICE[24659] chan_sip.c: SIP Transfer attempted with no appropriate bridged calls to transfer[/quote]

I have no other indication as the issue at hand (I don’t know why we’re seeing this warning but it’s screwing things up). However the impact to the users is that the queue member gets a ringing to their SIP phone and go to answer, the call seems to try and bridge but they cannot hear the calling extension. The calling extension, cannot hear either and seems to still be ringing.

We see the warning 2-3 times minimum a day. Essentially the caller has to hang up and try again, at which point it typically works.

I need to understand (advice) on possible causes or places to look for resolving this. Typically warnings aren’t an issue, but this has a real world impact.


We have queues.conf setup like so:

All of our SIP extensions are statically defined in SIP.CONF like so:

We repeat several of the “service1” style sip client setups, but change the names, etc but they’re all configed the same way so didn’t bother to list all of them.

Then extensions.conf is like so:


Simple first thing is to upgrade. 1.4.23 was buggy.


I see 1.4.24 is out now. I’ll update and see what happens. I should have payed attention to the releases, but man I feel like I just pulled down the latest 1.4.x only a couple weeks ago, these versions move quick around here.

Thanks for the feedback.


how’d you go, im getting Unexpected control subclass 20 when I call playback(invalid).