you can't make anyone answer the phone.
Well you can try asking them for it? Haha or asking a friend of theirs. Phone books are usually good to look in too.
Yes, it is unlawful to bug a phone.
There's no 'surefire' way any more. In the early days of mobile phones, different carriers were allocated certain 'block' of numbers. For example, all Vodaphone numbers would start 079xx perhaps....
You can call them. If they answer, the phone is on.