Unable to forward voice or dtmf

Hi,

dont know why but sometimes my got message like this:

[quote][Sep 20 12:12:16] WARNING[24820]: app_dial.c:741 wait_for_answer: Unable to forward voice or dtmf
[/quote]
And cant make a call…

[quote]Auto fallthrough, channel ‘SIP/122-b6889880’ status is ‘CHANUNAVAIL’
[/quote]

When reconnect trunk or restart asterisk it works again.
How can i diagnose it?

Greetings

I have the same problem when calling to 09xx numbers in holland !

But not with all numbers.