[Asterisk-Users] Already on the phone?

[Michael Ulitskiy]

Hi,

I'm wondering if there's a way within a dialplan or AGI to find out 
if an extension (SIP client) is already in use and the 
person is already on the phone?
By default the channel is assumed available and callwaiting tone
is transmitted to the called extension. AFAIK there's no way to turn
off callwaiting from within the dialplan. 
I need to avoid the callwaiting behavior in some cases and pass the
call to another extension if called extension is already in use. Is this
possible with asterisk? 
I've tried chanisavail application, but since callwaiting is enabled it
always returns true.
Thanks.

Michael