if they have call waiting activated on their phone, you'll know they're on the phone when it beeps (toot..toot.toot) when you ring them, without a dialing tone. OR they may just have the phone set to "go to voicemail when busy" in which case the phone will go straight to voicemail, however you wont know theyre on the phone because it may just be switched off.
It depends on their cellphone, most cellphones still ring normally and then goes to voice mail if they don't beep over. Usually going straight to voice mail without ringing means their phone is off or they rejected the call.
You cant know. Like mandy said, with most cellphones it will continue to ring. The busy signal doesnt really exist any more.