I have a running asterisk instalation with a couple of extensions. Yesterday we've discovered a strange problem. I'm making an outgoing call (though a sip trunk of our telephony provider). The remote party answers and tries to transfer the call to another extension. Then something strange happens: our asterisk starts playing our own "music on hold" to my extension (I suppose it shoud have forwarded me whatever the remote party was sending in its RTP stream). From then on the call is unrecoverable.
This is the diagram of a traffic flow between the asterisk and our telephony provider. The "music on hold" starts when our asterisk receives the INVITE from the provider.http://postimage.org/image/lmgpoe5z9/
Please, help me understand what is going on. How could the remote party make asterisk play his own music on hold? Is that my misconfiguration?
P.S. My guess would be that the remote party might be sending a DTMF code that makes asterisk behave that way. However dtmf is configured as "inband" in asterisk. Is there a way to detect such dtmf codes with wireshark (or in some other way)?