asterisk users - Jan 2010 - How to test if a telephone is busy now?

hi, all

i want to test if a telephone is busy now in agi php script?

Could you tell me how to do that judgement?


example?

if( ivan is not busy)
{
     $agi -> exec_dial("SIP","ivan");
}
else if (test is not busy)
{
    $agi -> exec_dial("SIP","test");
}

Thanks very much!

-- 
Best regards,
Sucan