[Asterisk-Users] detect SIP phone availability before dialing

[bman at host.sk]

Hello,

 I need to detect availability of SIP phone before dialing. I need to
know if phone is 
 BUSY, CHANUNAVAIL before dialing. If phone is "free", then I will dial
it.

 I need for automatic callback (.call files), but I need to know if it
is available both
 SIP phones before calling.