Check the markup if busy or unanswered

Hello, I am implementing a telephone server asterisk in PHPAGI and I would check that if I call is busy or does not answer in time, I go to another phone.

How could I?

My implementation is:

if ($ r1 ['Code'] == 'xxxx')
{
$ AGI-> exec ('Dial', "SIP/xxxxxxxxxxxx/00xxXXXXXXXXX,". $seconds. "hr");
$ AGI-> hangup ();
}
else
{
$ agi-> wav ('Error');
$ AGI-> hangup ();
}

00xxXXXXXXXXX is the phone number to which the call is made, but I'd put another alternative.


Thank you.

put it in a loop with a timestamp to check duration?