Phone off hook (busy line) detection

I didn’t have much luck finding anything in the forums, so I thought I’d ask. How can Asterisk tell if a Zap line is busy (someone is on it)? I am still in the learning stages of Asterisk and have a cheap IP phone (Budgetone 100) hooked up to a Asterisk server with an X100P card. Today I started to dial out using this code:[code][globals]

ignorepat => 9
exten => _9NXXXXXX,1,Dial(${OUTBOUNDTRUNK}/${EXTEN:1})
exten => _9NXXXXXX,2,Congestion()
exten => _9NXXXXXX,102,Congestion()
exten => 911,1,Dial(${OUTBOUNDTRUNK}/911)
exten => 9911,1,Dial(${OUTBOUNDTRUNK}/911)[/code]

I’m pretty certain I accidently broke in on someone’s conversation. I had thought this would make sure that didn’t happen. Is there a way for Asterisk to detect if the line is in use?